There is a single measure of fitness more strongly tied to lifespan than almost any other, and most people have never had it measured. It is your cardiorespiratory fitness, closely related to your VO2 max: how effectively your body takes in and uses oxygen during hard effort. The evidence for its importance is remarkable. In a study of more than 122,000 people undergoing treadmill testing, higher cardiorespiratory fitness was associated with lower mortality, and critically, there was no observed upper limit to the benefit (Mandsager et al., JAMA Network Open, 2018). The fitter people were, the longer they tended to live, all the way up the scale. The difference in mortality risk between the least fit and the most fit was enormous, comparable to or greater than the risk associated with major conditions like coronary artery disease or diabetes. What makes this encouraging rather than intimidating is that cardiorespiratory fitness responds quickly to training. Unlike some health markers that move slowly, this one can improve within weeks, and it responds at essentially any age. You are not stuck with the number you have now. And you do not need hours of steady cardio to move it. Short, hard efforts are remarkably effective. High-intensity interval training, which means alternating short bursts of hard effort with periods of recovery, has been shown to improve VO2 max meaningfully, even in low-volume forms (Wen et al., Journal of Science and Medicine in Sport, 2019). The great appeal is time efficiency: you can generate a strong cardiovascular stimulus in a fraction of the time that long steady sessions require. The reason intervals work so well comes down to intensity. Pushing close to your maximum forces your heart and muscles to adapt in ways that comfortable, easy efforts do not fully stimulate. Brief exposure to hard effort, repeated, drives the adaptations that raise your fitness ceiling. This does not mean easy movement is worthless. Far from it. Intervals are the hard end of a complete picture, best used a couple of times per week alongside more frequent easy activity, which we cover separately. The two work together rather than competing. Adding intensity safely is mostly a matter of restraint at the start. Begin with a small number of short, hard efforts, each followed by full recovery. You do not need many to get a benefit. The efforts themselves should be genuinely hard, but, just as importantly, the recovery between them should be genuinely easy. Both halves of the interval matter. For most people, twice a week is plenty. Interval training is a potent stimulus, and doing more is not better here; it mainly adds fatigue and injury risk without proportional benefit. Build up gradually over weeks rather than jumping into large volumes of maximal effort. There is an important safety caveat. All-out effort places heavy demand on the cardiovascular system. If you are new to exercise, older, or have any heart concern or risk factors, you should be cleared by a doctor before doing maximal-intensity work. This is one area where caution genuinely matters. A simple way to begin is to include a few short, hard efforts in a walk or ride this week, with easy recovery between them, and notice how quickly your ability to recover between efforts improves. That improving recovery is a direct, felt sign that your fitness is responding. Cardiorespiratory fitness is one of the most powerful predictors of a long life that you can actually change, and change quickly. A small amount of well-managed high-intensity work, added to a base of easier movement, is one of the highest-return investments you can make in how long and how well you live.
Safety
This article is educational and not medical advice. High-intensity exercise carries cardiovascular risk for some people. If you are older, new to exercise, or have any heart condition or risk factors, consult your doctor before beginning high-intensity training.
References
Mandsager K et al., JAMA Netw Open, 2018 (PMID 30646252). Wen D et al., J Sci Med Sport, 2019 (PMID 30733142).