Aerobic capacity

Aerobic capacity is the size of your oxygen-powered engine. Its headline number is VO2 max — the maximum rate at which you can consume oxygen — but the full picture includes how much blood your heart moves per beat, how densely your muscles are supplied with capillaries, and how many mitochondria those muscles carry.

It matters because it is one of the strongest single predictors of long-term health that exists. Large cohort studies repeatedly find that low cardiorespiratory fitness carries a mortality risk comparable to or greater than smoking, diabetes or high blood pressure — and unlike your genes, it is modifiable at any age.

In practice it improves through a large base of easy aerobic work, which builds the peripheral machinery, topped with a smaller amount of hard interval work that pushes the central limit. Both parts matter; most people under-do the easy volume.