Exercise is undoubtedly ‘good’ for you, and is associated with improved health outcomes, particularly if maintained into older age. Ideally then, it would seem that the ‘perfect’ exercise program (if health and longevity is the goal) would be one which is sustainable in the long term and not necessarily one which produces rapid results prior to an equally dramatic crash and burn.
Exercise training always involves consideration of ‘risk’ and ‘reward’. There are several risks inherent in exercise – injury, overtraining, cardiac arrest, fatigue, immune dysfunction, falling off your bike, getting hit by a car etc. Although these are potentially catastrophic, in most cases the probability of these occurring is relatively small. However, there are undoubtedly characteristics peculiar to the individual which increase the specific risk (eg. knock knees or a habit of riding your bike around the country lanes in the dark and with no lights).
Happily however, there are rewards to be had in terms of beneficial physiological adaptation, enjoyment, mental health benefits or sociability. These are not guaranteed, but generally (unless you do something really idiotic) the likelihood of reward is greater than the likelihood of negative outcomes, and the ‘risk’ is worth taking on the balance of probability.
Now, at the risk of being a little speculative, I would suggest that the risk of exercise induced negative outcomes increases with exercise load. That sounds fairly intuitive, but I’d also suggest risk of negative outcome also increases with intensity (rationale for this statement can be found in this paper – https://www.tandfonline.com/doi/full/10.1080/15438627.2021.1906672). Of course, there is also the possibility that this greater degree of risk is accompanied by greater potential reward (high intensity nearly always ‘wins’, at least in short term exercise studies).
If for some reason I had to get fit quickly, I wouldn’t hesitate to throw a lot of high intensity training at my body. Yes, there is a risk, but on balance I conclude that I’m most likely to experience an overall benefit. But… what if the goal is to be active for a lifetime? I’m going to largely deal in hypotheticals here, but I have seen a study suggesting that 30% of regular runners can be expected to experience a lower limb injury in a given 2-year period. The odds there still sound reasonable – there is a 70% likelihood that you won’t be injured. However, what if you carry on for 4 years? 6 years? Undoubtedly its much more complex and individuals will vary in their own degree of risk due to their peculiarities, but if everyone is the same then over 6 years ~90% of runners can expect some form of injury. These injuries are not necessarily serious, but a strong predictor of future injury is precious injury, so you can imagine how things might progress. Sustaining a running habit across a lifetime without experiencing problems therefore looks vanishingly unlikely.
The key here is that risks are cumulative, and some negative event that appears unlikely in the short term may become almost inevitable if an activity is performed for long enough. So if this is correct, are there any implications for us exercisers? I think there are. If you want a lifetime of sustainable exercise you may need to play it safe / conservative and avoid high loads (and particularly intensities) of exercise. If the goal is high performance you will need to occasionally roll the dice. However, don’t be surprised if eventually you go bust.