VO2 max is the maximum rate at which your body can use oxygen during intense exercise — a single number capturing the whole chain from lungs to heart to mitochondria. It's the best single measure of cardiorespiratory fitness, and it turns out to be an extraordinarily strong predictor of how long you'll live, rivaling or exceeding traditional risk factors like smoking and hypertension.
Training it well takes two ingredients. Zone 2 — sustained low-intensity work where you can still hold a conversation — builds mitochondrial density and fat oxidation, and it's where most endurance volume should sit. High-intensity intervals near your ceiling raise the ceiling itself. A common structure is a majority of easy Zone 2 volume plus one or two hard interval sessions weekly. The mistake most people make is training everything at a medium intensity that's too hard to accumulate volume and too easy to raise the ceiling.