Cardio is unambiguously good for you. But one of the more counterintuitive findings in exercise science is that more cardio is not endlessly better. The relationship between endurance exercise and health looks less like a straight line that keeps climbing and more like a curve that rises steeply, then flattens, and at the far extreme may even bend slightly the other way. This matters because the dominant cultural message is that more is always better, that if some running is good, a marathon must be better, and an ultramarathon better still. The evidence suggests a more nuanced picture, where moderate amounts of endurance activity deliver most of the available benefit, and very high volumes add relatively little while potentially introducing some downsides. One example concerns heart rhythm. Studies of lifelong, very high-volume endurance athletes have found somewhat higher rates of atrial fibrillation, a disturbance of the heart's rhythm, compared with the general population (Abdulla and Nielsen, Europace, 2009). This is an association drawn from observational data, and it applies to the extreme end of endurance training, not to ordinary exercisers. It is not a reason to avoid cardio. It is a reason to question the assumption that more is always the answer. It is worth being clear about proportion here. The large majority of people are not overdoing endurance exercise; if anything, most do too little. This information is not meant to discourage activity. It is meant to give the smaller group grinding out very high mileage an honest, complete picture rather than a one-sided story of unlimited benefit. So where is the sweet spot? Encouragingly, a great deal of benefit is available at very accessible levels. A meta-analysis of fifteen international cohorts found that all-cause mortality risk continued to decrease with more daily steps, with the benefit leveling off at approximately 6,000 to 8,000 steps per day for older adults and somewhat higher for younger adults (Paluch et al., Lancet Public Health, 2022). In other words, even walking delivers substantial health returns, and you do not need to run marathons to capture most of the benefit. Beyond a certain point, the additional benefit per additional mile flattens out. Easy, steady cardio builds your aerobic base and supports your health with very little downside, and this kind of comfortable movement is the bulk of what most people need. Combined with a small amount of higher-intensity work for your fitness ceiling, it covers almost the entire picture of cardiovascular health for the average person. This reframes very high mileage as a choice rather than a requirement. If you run long distances because you love it or because you compete, that is a completely valid reason. The key is to know which reason applies to you. Pursuing extreme endurance for the love of the sport is different from believing you must do it for your health, and the health case simply does not require it. Building the right amount into your life is straightforward. Move easily and often, working toward something in the range of 7,000 to 8,000 steps a day as a general target, and add a couple of harder efforts a week for your fitness ceiling. For most people, that combination is the whole recipe. If you love distance running, do it, but go in understanding that the extremes carry tradeoffs, and build your volume up slowly to protect your joints and your heart. A useful thing to observe in yourself is whether your cardio is serving your health or quietly becoming a compulsion. That distinction matters, both physically and psychologically. The goal is a body that lasts and functions well, not a mileage number to point to. Match the dose to the goal. For health, moderate and consistent wins. For sport, go further with open eyes. Either way, the science frees you from the belief that you must run yourself into the ground to be well.
Safety
This article is educational and not medical advice. If you have any heart condition, joint problem, or other health concern, consult your doctor about the appropriate amount and type of exercise for you.
References
Paluch AE et al., Lancet Public Health, 2022 (PMID 35247352). Abdulla J, Nielsen JR, Europace, 2009 (PMID 19633305).