Zone 2 training: build your engine slowly.

You run three evenings a week, always flat out, until your lungs give up. Then you read about zone 2 training, the easy cardio, and it sounds like an excuse not to work hard.

I see it often in the managers I work with. They push hard at the office and on the treadmill, and a month on they're more worn out than fit. Building stamina usually means going slower than you think.

How do you calculate zone 2?

Breathing first, then heart rate. Aim for three or four out of ten, checked with the talk test.

• The talk test. Say a long sentence out loud as you go. If you have to break it to breathe, ease off. If you could sing it, speed up a little.
• Heart rate. Take your age from 220 for your estimated maximum. Zone 2 is 65 to 75 per cent of that, so at fifty roughly 110 to 128 beats a minute. The formula can be out by ten or so: if your breathing and the number disagree, trust your breathing.
• The warning sign. Panting means you've left the zone and are burning mainly sugar.

Why easy cardio builds the engine

Inside every muscle are mitochondria, tiny power stations that use oxygen to turn fat and sugar into energy. The more you have, the more energy you make without gasping, and the better you handle sugar: with too few, it lingers in your blood and your body grows less responsive to insulin.

An easy pace works the right fibres. Slow-twitch fibres are packed with mitochondria and burn mostly fat. In zone 2 they work for a long time, and your body builds new power stations: mitochondrial proteins can double.

Your engine winds down when it sits idle. After thirty, aerobic capacity drops slowly, by up to about a third by sixty-five. Training can raise it by up to twenty or thirty per cent.

Your heart learns to save effort. It pumps more blood per beat, and resting heart rate and blood pressure tend to fall. In one comparison, after six weeks' training the same speed took less oxygen.

How much zone 2, when, and on what

Three or four times a week, thirty to sixty minutes. At least 150 minutes in total, with the benefit growing up to about 300. If your diary won't stretch, two fifteen-minute blocks count. One session's effect on your metabolism lasts about two days, so frequency matters.

What to use. Bike, exercise bike, rowing machine, pool, uphill walking. It's the pace that counts: the same bike can be zone 2 or a slog.

Zone 2 walking or running? Carrying extra kilos or have dodgy knees? Walk uphill, on an inclined treadmill if need be. If you run, drifting out of the zone at first is normal: walk a minute, then carry on.

Plus a dash of intensity. Short of time? Mix it up: lots of zone 2 and one short, hard session a week, if your body copes and your doctor agrees. More on that in the article on weights or cardio.

This week, put those three sessions in your diary like meetings, with a time. The first few will feel too slow. That's the right pace.