What is Zone 2 training?

Zone 2 training is one of those ideas that sounds almost too simple to be real. Train slowly. Go easy. Hold a conversation while you exercise. And somehow, over weeks and months, your fitness transforms.

I was sceptical too. Then I used Zone 2 training to go from a standing start to completing the Paris-Brest-Paris Audax — 1,200km in 142 hours. What I learned along the way upended 30 years of assumptions about how fitness actually works.

This page covers everything you need to understand about Zone 2: what it is, why it works, how to find your Zone 2, and what the science says.

The five heart rate zones

Zone 2 gets its name from a five-zone model of exercise intensity, where Zone 1 is the easiest and Zone 5 is the hardest. The zones are defined by percentage of maximum heart rate:

Zone 2 sits at the top of the aerobic zone — hard enough to stress your aerobic system, easy enough to sustain for hours. It is the zone below your anaerobic threshold (also called lactate threshold), the point at which your body switches from primarily burning fat to primarily burning carbohydrates.

What actually happens in Zone 2

To understand why Zone 2 is so effective, you need to understand mitochondria — the tiny energy-producing structures inside your muscle cells.

When you exercise in Zone 2, your muscle cells are working hard enough that mitochondria are under sustained stress. Your body responds to this stress by making more mitochondria, and by making the ones you already have larger and more efficient. This process is called mitochondrial biogenesis.

More mitochondria means your muscles can produce more energy aerobically — using fat and oxygen rather than glucose. The practical results:

• You can sustain effort for longer before fatigue sets in
• You burn more fat at any given intensity
• You recover faster between sessions
• Your overall cardiovascular efficiency improves

"Zone 2 is the highest intensity at which mitochondria are being maximally stressed without crossing into anaerobic metabolism. It is the sweet spot for aerobic adaptation." — Dr. Iñigo San Millán, physiologist and performance director, UAE Team Emirates

The aerobic vs anaerobic divide

This is the insight that changed everything for me. Most amateur athletes spend most of their training time in Zones 3 and 4 — the "moderately hard" zone that feels productive but isn't particularly good for either aerobic development or high-end fitness.

Elite endurance athletes follow a very different pattern, known as polarised training:

• ~80% of training in Zones 1–2 (aerobic base work)
• ~20% of training in Zone 5 (true high-intensity intervals)
• Minimal time in Zones 3–4 (the "grey zone")

This is what I followed in my own training. Around 80% of every week's sessions were Zone 2. The rest was high-intensity intervals or strength work. Almost no Zone 3 or 4. It felt counterintuitive — but the results were unambiguous.

Zone 2 and fat metabolism

One of the most striking things from my lab testing was seeing my fat metabolism change over six months of Zone 2 training.

In my initial VO2Max test, even at low intensities I was burning mostly carbohydrates — surprising for someone who'd done endurance training for years. My hypothesis: a carb-heavy diet and evening snacking had trained my body to default to glucose as fuel.

Six months later, after consistent Zone 2 training, the same test showed dramatically improved fat oxidation across a much wider heart rate range. At lower intensities I was burning a much higher proportion and volume of fat — exactly what you want for endurance events, where glycogen is a limited resource but fat stores are effectively unlimited.

This is the metabolic adaptation that makes Zone 2 so powerful for endurance: your body learns to run on its most abundant fuel source.

Zone 2 and longevity

Zone 2 has moved beyond endurance sport into mainstream longevity medicine. Researchers and physicians including Peter Attia and Dr. Iñigo San Millán consider it one of the highest-leverage interventions for long-term health — not just fitness.

The reasons are well-grounded in metabolic science. Improved mitochondrial function is linked to reduced risk of metabolic disease, type 2 diabetes, cardiovascular disease, and cognitive decline. Better fat oxidation reduces visceral fat — the fat around your organs that drives inflammation and disease risk. Improved aerobic capacity (VO2 Max) is one of the strongest predictors of all-cause mortality in the research literature.

Put simply: Zone 2 training makes your cells more efficient, and cellular efficiency is closely tied to how well and how long you age.

How to find your Zone 2

There are four main methods for calculating your Zone 2 training range. I've tried all of them — here's what each involves and how they compare:

1. Percentage of maximum heart rate

Zone 2 = 60–70% of your maximum heart rate. You need to know (or estimate) your max HR. The simplest method, though the least precise.

2. Maffetone MAF 180 formula

Subtract your age from 180. Adjust by a few beats depending on your training history and health. This gives you the top of your Zone 2 range. I'm 51, so my starting point is 129 bpm.

3. Percentage of anaerobic threshold

Do a 30-minute best effort on a bike or running. Take your average heart rate from the final 20 minutes as your anaerobic threshold. Zone 2 is roughly 85–89% of that number. More accurate, but requires a hard effort test.

4. The talk test

The simplest and surprisingly accurate method. Warm up, then gradually increase intensity. When you can hold a conversation but it's slightly effortful — that's Zone 2. When completing a sentence becomes difficult, you've crossed into Zone 3.

For a personalised calculation, use the heart rate calculator — enter your age and it gives you your Zone 2 range using multiple methods.

Common Zone 2 mistakes

Going too hard

This is the most common mistake, and I made it myself early on. Zone 2 feels uncomfortably easy when you're used to training hard. If you can't hold a conversation, you're not in Zone 2 — you're in Zone 3, and missing the adaptation you're training for.

Ignoring the "grey zone"

Zones 3 and 4 aren't useless — but spending most of your training there means your easy sessions aren't building your aerobic base and your hard sessions aren't developing your VO2 Max. Most amateur athletes are stuck in the grey zone without realising it.

Expecting quick results

Mitochondrial adaptation takes weeks and months, not days. The early weeks of Zone 2 training feel frustratingly slow. Stay in the zone, keep the sessions consistent, and the results compound over time.

Only cycling or only running

Zone 2 adaptation is partly global (cardiovascular) and partly local (specific muscles). Adding variety — cycling, running, swimming, hiking — builds a broader aerobic base and keeps training sustainable.

What Zone 2 can (and can't) do

Zone 2 is not a magic bullet. It builds your aerobic base exceptionally well. It won't, on its own, develop your top-end speed or VO2 Max — that requires high-intensity work. The most effective training programmes combine the two: a large volume of Zone 2 with a small amount of Zone 5, and minimal time in between.

For general health and longevity, consistent Zone 2 work — even without the high-intensity component — delivers substantial benefits. For competitive endurance performance, the combination of both is where the real gains come.