VO2 Max.

VO2 max is the most oxygen your body can actually use in a minute, and it is the single number that best predicts how long you live. You raise it by spending time close to your ceiling — roughly 90% of max heart rate and above.

Getting there takes a couple of minutes of hard work, so the whole game is reaching that zone and then staying in it. Long intervals get you deep and hold you there. Short ones get you back there over and over without wrecking your legs. Both work. Doing the same one every week stops working.

Two sessions a week is plenty on top of lifting. A third only pays off if sleep and food are already handled.