SLOW DOWN TO SPEED UP; IMPROVING YOUR PERFORMANCE BY SLOWING DOWN

Why the easy runs need to actually flipping be easy! The physiology of what Zone 2 actually builds. And why most tactical athletes are stuck in the grey zone that fatigues them without making them fitter.

Introduction

In training for the military there’s a belief held by the high majority of people: the harder and faster you train, the better you’ll perform.

But research shows us there’s more to it than ‘go out and run as hard as you can each session’.

Think about this: a Royal Marines Recruit covering the 30-miler or a UKSF candidate completing the long walk. 

A long insertion followed by hard work at the other end. None of that gets done at a sprinter's speed. It gets done at a marathoner's pace, held for hours, sometimes days, sometimes across weeks.

That capacity doesn't come from doing everything hard.

It comes from building a large, efficient aerobic engine underneath the hard work. And the way you build that engine is by training slowly enough that the right adaptations actually get triggered.

Most tactical athletes never do this. They live in Zone 3. Hard enough to fatigue them. Not hard enough to force high-end adaptations. Not easy enough to develop the base. That's the trap this article is trying to get you out of.

What Zone 2 actually is - physiologically.

Zone 2 is the intensity at which your body is producing energy 'almost entirely aerobically', using oxygen and (largely) fat as fuel, with lactate production still low enough that your body clears it as fast as it's made (at least initially).

You can hold a conversation with sentences around 15 words at this intensity. Your heart rate sits somewhere around 60-70% of your maximum, though the exact figure varies by fitness level, age, climate etc. You're working, but you could it keep going for hours.

Cross above this zone/threshold, and lactate starts to accumulate faster than you can clear it. Your body shifts toward burning glucose over fat. You start to fatigue. The window where Zone 2 adaptations happen starts to close.

Table 1. The five training zones and what each one actually does. Zone 2 is the base. Zones 4 and 5 are the ceiling. Zone 3 is where most tactical athletes accidentally spend their weeks.

What Zone 2 training actually builds

The reason Zone 2 sits at the centre of every serious endurance programme, from world-tour cyclists to Norwegian cross-country skiers, is that it drives a specific set of adaptations that no other intensity produces as reliably.

Mitochondrial density

Mitochondria are the aerobic engines inside your muscle cells. They take fat and glucose and oxygen, and they turn them into ATP.

Zone 2 training increases both the number and the size of your mitochondria. More mitochondria means more energy produced aerobically at any given effort. Which means you can go harder for longer before you tip into anaerobic territory.

This adaptation happens best at Zone 2. Push harder and you don't get more of it. Go easier and you don't trigger enough of it. The signal window is specific.

Capillary density

Capillaries are the tiny blood vessels that surround your muscle fibres. They deliver oxygen and fuel in, and clear waste products out.

Zone 2 work drives the formation of new capillaries around trained muscle. More capillaries means better delivery, better clearance, more work per unit of oxygen. The muscle gets more efficient.

Stroke volume and cardiac efficiency

Stroke volume is how much blood your heart pumps with each beat. Zone 2 work, held for extended durations, drives the heart to become a stronger, more efficient pump.

Higher stroke volume means the heart doesn't have to beat as often to deliver the same amount of blood. Resting heart rate drops. Heart rate at any given pace drops. You can hold higher outputs for longer before your heart maxes out.

Fat oxidation

At Zone 2 intensities, your body is heavily reliant on fat as fuel. Consistent Zone 2 training teaches the body to burn fat more effectively even at higher intensities. Which spares glycogen. Which delays fatigue.

For a tactical athlete moving under load for hours on end, this is certainly a high operationally relevant adaptation you can train. A soldier with better fat oxidation runs deeper into their fuel reserves before they hit the wall. And they recover faster from the effort.

Vagal tone

Zone 2 work increases parasympathetic (vagal) tone, which shows up as better heart rate variability, faster recovery between efforts, and greater resilience to stress. The Stress and the Tactical Athlete article on the website walks through why this matters for total load management.

The 80/20 rule: what the research actually tells us

The most-studied training distribution in endurance science is called polarised training, often summarised as the 80/20 rule.

Roughly 80% of total training volume is done at low intensity, in Zone 2. Around 20% is done at high intensity, in Zones 4 and 5. The middle ground gets 'almost' nothing.

Seiler's original work on this in Scandinavian endurance athletes (2006), followed by decades of subsequent research across running, cycling, rowing, cross-country skiing and triathlon, has consistently shown that elite endurance athletes, across sports, converge on roughly this distribution (Seiler & Kjerland 2006; Kenneally et al. 2018; Stöggl & Sperlich 2015).

They don't spend most of their time in the middle. They spend most of their time easy, and a specific chunk of their time hard.

Why does this work?

Because Zone 2 and Zone 4-5 drive different adaptations. Zone 2 builds the base. Zone 4-5 develops the ceiling. Zone 3 does neither particularly well: too hard to be a base session, too easy to be a ceiling session. And crucially, Zone 3 accumulates fatigue at a rate close to Zone 4-5 without producing the same adaptations.

For a soldier building conditioning across weeks and months, this matters. Every training minute has a fatigue cost and an adaptation return. Zone 2 is the highest adaptation-to-fatigue ratio available. Zone 4-5 is worth the cost when applied in the right dose. Zone 3 is the worst of both worlds.

How to actually find your Zone 2

Five methods, from most accurate to quickest.

Table 2. Methods for finding your Zone 2. The lab test is the gold standard. The talk test is probably what you'll actually use.

A practical note. Most wearables now estimate your zones automatically. They're better than nothing, but they can drift by 10-15% either way. If your device says you're in Zone 2 but you can't hold a sentence, trust your mouth over your watch. If it says you're in Zone 3 but you feel comfortable and can breathe through your nose, you're probably fine. (This is without us going into how wearable tech is often wrong or has a high enough standard deviation that it'll throw your training sessions into chaos if that's the only method you're relying on).

How to structure Zone 2 into a weekly plan

For a tactical athlete building toward selection, deployment or a heavy operational period, a workable weekly template looks like this.

• Two to three Zone 2 sessions per week. Forty-five to seventy-five minutes each. Running, rucking, cycling or rowing. Vary the modality if your legs need a break.

• One threshold session per week. Four by four minutes hard, or two by ten minutes at threshold pace. This is your Zone 4 dose.

• One VO2max session every one to two weeks. Six by 800m, or five by three minutes at the pace you could just about hold for six. Small dose, real cost, don't overdo it.

• One long run or slow tabbing session at the weekend. Ninety minutes or more, low Zone 2 throughout.

The rest of your conditioning time (if any) can be recovery walks or mobility. Just watch out that you don't fill it with unnecessary junk miles or more medium-hard work.

Why trying to just run harder all the time makes you slower

There's a specific failure mode that catches most soldiers trying to improve their run times.

They train hard three or four times a week. Every session is roughly the same intensity: uncomfortable, sustainable for the duration, over the top of Zone 2 but nowhere near the top of Zone 4. Initially, their times improve because any structured training will beat no training hands down.

Then they plateau. Usually inside six to eight weeks.

What's happened is that they've built the middle of the intensity spectrum and neglected both ends.

The aerobic base isn't developing because they never train slow enough. The high-end ceiling isn't developing because they never train hard enough. Every session is a solid Zone 3 grind that's fatiguing but not adaptive.

The fix is counterintuitive. You slow down.

You take the majority of your running back to a pace where you can hold a conversation. It'll feel embarrassingly slow for the first three or four weeks. Then, quietly, the same pace starts producing a lower heart rate. Then the pace itself starts to creep up while the heart rate stays constant. By month three, you're running faster at Zone 2 than you used to run at your grind pace.

At that point, the hard sessions start doing what they're supposed to do. Because now you have a base to stack them on top of.

What it feels like across the first three months

The honest experience of switching to a Zone 2-heavy training approach, for someone who hasn't done it before, goes something like this.

• Weeks 1-4: Frustrating. You feel like you're going too slowly. Your ego doesn't like it. You'll question the plan, message your coach to see if they actually know what the hell they're talking about... Stick with it.

• Weeks 4-8: The pace at which you can hold Zone 2 starts to creep up. Same heart rate, faster pace. You start to feel like you're being paid back for your patience.

• Weeks 8-12: Your hard sessions start feeling different. The threshold work is easier to hit because the base is now supporting it. Your run times drop even though most of your training got easier.

• Beyond 12 weeks: You've become a far superior aerobic athlete. Hard sessions cost less. Recovery is faster. Load carriage feels different. You'll wonder how you spent years grinding without ever building the engine that would have made the grinding pay off (sometimes at this point it's nice for your coach to hear from you...)

Nothing about this is fast. All of it works.

You can't stack a tall pyramid on a narrow base

This is the sentence to really ingrain in your mind from this whole article.

Every high-intensity capacity you have (your two-mile run, your intervals, your operational sprint work, your recovery between hard efforts) sits on top of the aerobic base underneath. Widen the base and everything above it rises. Try to build the tip without the base and you'll cap out earlier and break more often.

Zone 2 is how you widen the base. Slow enough to be genuinely aerobic. Long enough to accumulate the stimulus. Frequent enough that the adaptations build. The Stoic Conditioning app puts this into every conditioning section of our programming by default, because it's the single most under-prioritised quality in tactical training and the one that most reliably moves the numbers on everything else.

Bottom line

Slow down to speed up. It sounds like a paradox and it isn't.

The tactical athletes who eventually run the fastest two-milers, hold the highest SCA paces, and complete the hardest tabs with something left in the tank are the ones who spent months of their easy work at genuinely easy paces. That's what allowed their hard work to do its job.

Five takeaways.

• Two to three Zone 2 sessions a week, 45-75 minutes each. Conversational pace throughout.

• Use the talk test. If you can't hold a sentence, you're too hard. If you can shout across a road, you're too easy.

• Don't ignore the hard sessions. One threshold, one occasional VO2max. Just make sure the easy work stays easy.

• Give it 12 weeks before you judge whether it's working. The first month usually feels wrong.

• Zone 2 is protective as well as adaptive. Low injury risk, low fatigue cost, high mitochondrial return. It's the safest way to increase weekly training volume.

Make the easy runs easy. Make the hard runs hard. Get out of the middle.

Zone 2 training for tactical athletes: frequently asked questions

What is Zone 2 training?

Zone 2 is a moderate-intensity training zone. Roughly 60-70% of maximum heart rate, at a pace where you can hold a full sentence without gasping. It targets the aerobic energy system directly, building mitochondrial density, capillary networks, stroke volume and fat oxidation. For tactical athletes, Zone 2 work should make up around 80% of weekly conditioning time.

How do I find my Zone 2 heart rate?

Three practical methods. Best: a lactate threshold test in a lab. Middle: the talk test (a full sentence, 8-12 words, without gasping). Quickest: 220 minus your age gives an approximate max HR, and 60-70% of that figure is a rough Zone 2 range. The talk test is more reliable than the maths for most people.

If you're using a wearable, use it as a guide, not as the truth. Devices drift. Your mouth doesn't lie.

How long should a Zone 2 session be?

Forty-five to seventy-five minutes is the productive range. Below thirty minutes, the adaptations don't accumulate much. Above ninety minutes, you start hitting diminishing returns unless you're training for very long-duration events.

Two to four sessions of forty-five to seventy-five minutes a week delivers the full benefit for most tactical athletes.

How often should I do Zone 2 training?

Three to five sessions a week is the target for a candidate building toward selection. The 80/20 rule, popularised by Seiler and others, uses around 80% of total weekly training volume in Zone 2. For a soldier doing eight hours of conditioning per week, that's roughly six and a half hours easy and one and a half hours hard.

Why is Zone 2 so important for soldiers?

Three reasons.

Operational tasks (loaded marches, sustained movement, casualty drag) sit largely in the aerobic energy system. A stronger aerobic base directly improves the ability to do them for longer.

Zone 2 training builds the mitochondrial and capillary density that supports recovery between higher-intensity efforts. So the hard bits get less costly.

And it delivers these benefits at low injury risk and low fatigue cost, which means it doesn't interfere with the strength and skill training that has to happen alongside it.

Can you do too much Zone 2?

Yes, but it's hard. The main failure mode is volume creeping above what your work schedule, sleep and nutrition can support. Zone 2 is low intensity, but it isn't free.

The bigger issue for most soldiers is doing too little Zone 2 and too much middle-ground work. If your training's dominated by sessions where you can't hold a conversation but aren't truly going hard either, that's the problem to fix first.

What if I can't keep my heart rate in Zone 2 when running?

It means your aerobic base hasn't been developed yet.

The fix is patience. Slow down to a walk if necessary. Then a slow jog. Then a faster jog. Build the heart's stroke volume and mitochondrial density over weeks. Six to twelve weeks of disciplined Zone 2 work is enough to drop most people's running heart rate at the same pace by 5-15 bpm.

The first month feels embarrassingly slow. The second month feels normal. The third month feels fast.

How long does it take to see results from Zone 2 training?

Most cardiovascular adaptations appear within 4-6 weeks. Measurable drops in heart rate at the same pace. Easier breathing during efforts you used to struggle with. Mitochondrial and capillary adaptations continue to accumulate over 3-6 months.

The gains from Zone 2 are slower to appear than the gains from high-intensity work. They're also more durable, and they raise the ceiling of what your high-intensity work can produce.

What's the difference between Zone 2 and LISS or steady-state cardio?

They're essentially the same thing under different names. LISS is the bodybuilding-derived term. Zone 2 is the heart-rate-based term used in endurance and tactical-athlete literature. Steady-state cardio is the generic descriptor. All three describe sustained moderate-intensity work below the lactate threshold. The principles in this article apply identically regardless of which term you prefer.

Can I do Zone 2 on a rower or bike instead of running?

Yes, and there's a strong argument for varying the modality. Cycling and rowing are lower-impact than running. They allow more total Zone 2 volume without the orthopaedic cost. They're also useful when running is contraindicated by injury.

The catch is that the central adaptations (heart, lungs, mitochondria) transfer reasonably well across modalities. The peripheral specificity (leg mechanics, foot durability, running economy) doesn't. If your test is a run, most of your Zone 2 should still be running. Cross-train the rest.

Further reading

If you want to go deeper on the qualities that sit alongside Zone 2 in a tactical athlete's programme:

• Improving Tabbing Performance - Zone 2 is the base of load-carriage endurance too.

• Stress and the Tactical Athlete - why Zone 2 is the most stress-friendly conditioning you can do when total load is high.

• Strength Training Does Not Make You Slow - the strength case for endurance-focused candidates.

References

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Billat VL, et al. Training and bioenergetic characteristics in elite male and female Kenyan runners. Medicine & Science in Sports & Exercise, 35(2):297-304, 2003.

Hickson RC, et al. Potential for strength and endurance training to amplify endurance performance. Journal of Applied Physiology, 65(5):2285-2290, 1988.

Kenneally M, Casado A, Santos-Concejero J. The effect of periodisation and training intensity distribution on middle- and long-distance running performance: a systematic review. International Journal of Sports Physiology and Performance, 13(9):1114-1121, 2018.

San-Millán I, Brooks GA. Assessment of metabolic flexibility by means of measuring blood lactate, fat, and carbohydrate oxidation responses to exercise in professional endurance athletes and less-fit individuals. Sports Medicine, 48(2):467-479, 2018.

Seiler S, Kjerland GO. Quantifying training intensity distribution in elite endurance athletes: is there evidence for an 'optimal' distribution? Scandinavian Journal of Medicine & Science in Sports, 16(1):49-56, 2006.

Stöggl T, Sperlich B. The training intensity distribution among well-trained and elite endurance athletes. Frontiers in Physiology, 6:295, 2015.

Swain DP, Franklin BA. VO2 reserve and the minimal intensity for improving cardiorespiratory fitness. Medicine & Science in Sports & Exercise, 34(1):152-157, 2002.