Zone 2 and VO2 max, the actual metrics that matter

Two cardio numbers will tell you more about how long you'll be moving well than almost anything else on a wearable. Zone 2 minutes a week. VO2 max.

Most men over 40 ignore both. They run hard when they should run easy, and they never run hard enough to push the ceiling. The result is a comfortable middle that builds neither base nor peak.

I made this mistake for a decade.

What Zone 2 actually is

Zone 2 is conversational pace. You can hold a sentence without gulping. You couldn't sing a chorus, but you could discuss the weather without distress.

In heart-rate terms, it's roughly 60 to 70% of your max heart rate. The exact percentage matters less than the feel. If you can't talk, you're too high. If you can recite a poem, you're too low.

The physiology, in plain English:

• Zone 2 trains your aerobic base, the slow-twitch fibres and the mitochondria inside them
• It improves your body's ability to burn fat as fuel, which spares glycogen for harder efforts
• It builds capillary density, so blood and oxygen reach muscle more efficiently
• It stresses the heart enough to grow stroke volume, with low joint and recovery cost

Zone 2 is the pace your old self called "too easy to count." It's the pace that quietly compounds for years.

Why VO2 max matters separately

VO2 max is your maximum rate of oxygen uptake, measured in millilitres per kilo per minute. It's a ceiling number. It tells you how high your engine can rev when fully extended.

The mortality data here is consistent across multiple large cohorts. Higher VO2 max correlates with lower all-cause mortality. The size of the effect is large, comparable to (and sometimes exceeding) the effect of not smoking. This isn't a fringe finding. It's one of the most replicated relationships in exercise epidemiology.

The catch: VO2 max declines with age, roughly 10% per decade after 30 if you do nothing. With training, you can hold it flat or improve it well into your sixties.

The simple weekly structure

The training that builds both numbers isn't complicated. The literature on aerobic development converges on roughly this:

• 80% of cardio time at Zone 2 (easy, conversational)
• 20% at Zone 4 or 5 (hard, intervals)
• Total: three to five cardio sessions a week

For an over-40 man with limited time, a workable week looks like:

• Two Zone 2 sessions of 30 to 60 minutes (cycle, run, brisk hike, rower)
• One interval session (4 to 6 reps of 4 minutes hard with 3 minutes easy, or similar)
• Optional third Zone 2 session if you've got a Saturday morning to spend

The hard session is what pushes the ceiling. The easy sessions are what build the floor that lets the hard session matter.

The middle that doesn't pay

Most men train in what coaches call "the grey zone." Hard enough to feel like you worked. Not hard enough to drive the high-end adaptations. Not easy enough to build the aerobic base.

It feels productive. It isn't. The grey zone gives you fatigue without specificity. You need both ends, not the middle.

The honest test: if you finish every cardio session feeling moderately tired but not destroyed, you're probably grey-zoning. The fix is uncomfortable. Make the easy days easier. Make the hard days harder.

What I track

I run two numbers regularly:

• Weekly Zone 2 minutes (target: 150 to 240, depending on the season)
• Estimated VO2 max from my Garmin (a reasonable proxy, not a lab test)

The Garmin number isn't lab-grade, but the trend is honest. If it's drifting up, my training is working. If it's drifting down, something is off (sleep, intensity distribution, life stress).

Once a year I get a proper test through a sports physiologist. The lab number anchors the wearable.

How to start if you're starting

If you've been mostly grey-zoning or mostly sedentary:

• Spend a month at Zone 2 only, three sessions a week, 30 minutes each
• Use a heart rate monitor (chest strap if you can, wrist-based if not)
• Resist the urge to "spice it up" for at least four weeks
• Add one interval session in week five

The first month feels slow. It is slow. That's the point. You're laying track.

A note on the tools

You don't need a Garmin. Any heart rate monitor and a bit of arithmetic gets you 95% of the way. Your max heart rate, roughly, is 220 minus your age. Take 60 to 70% of that for your Zone 2 range. It's a rough estimate, but it's good enough to start.

The wearable is a convenience, not a prerequisite. The discipline is.

(For anyone with known cardiovascular issues, get cleared by your GP before doing high-intensity intervals. The base work is generally safe, but the harder sessions deserve a check.)

Easy days easy. Hard days hard.