VO₂max is the most responsive fitness marker there is at the start and one of the most stubborn later. The shape of that curve is the thing worth understanding before you commit to a training block.
The timeline
Weeks 1 to 3. Plenty happening, little of it visible in this number. Plasma volume expands, heart rate at a given pace falls, and sessions start feeling easier. Your VO₂max estimate may not move at all.
Weeks 4 to 6. The first measurable change for most people. Stroke volume has begun adapting and the effect is large enough to clear the noise in an estimate.
Months 2 to 6. The steep part. An untrained person doing consistent mixed training commonly picks up 15 to 20 percent across this window, and it is the most rewarding period of endurance training you will ever have.
After six months. Flattening, hard. Gains become incremental and require more work for less return. Trained athletes fight for two or three percent a year, which is why the marginal obsessions start at this point.
What moves it fastest
Intervals near maximum. Three to five minute efforts at close to your ceiling, with recovery between, are the most potent single stimulus. Once or twice a week is plenty.
Easy aerobic volume. The larger and slower half of the equation, driving mitochondrial and capillary adaptations. Most people do far too little of it and run it too hard, which we put numbers on in elites train easier than you do.
The ratio between them. Roughly 80 percent easy, 20 percent hard is the pattern that shows up across endurance sports. Getting the easy zone right first is zone 2 heart rate, and why yours is probably wrong.
Why yours might be stuck
Everything is moderate. The most common pattern: too hard to recover from, too easy to drive adaptation.
Not enough total volume. Three easy runs a week has a ceiling and you will find it.
Under-recovering. Adaptation happens between sessions. A block that never lets you absorb the work produces fatigue rather than fitness, covered in the signs of overtraining.
The estimate is not updating. Boringly common on Apple Watch, which needs a specific kind of outdoor effort: Apple Watch cardio fitness not updating.
The protocol is in how to increase your VO₂max, and the reverse is how long it takes to lose fitness.