Comment by Mathnerd314

The relationship isn't linear, you can see in https://bjsm.bmj.com/content/54/20/1195#sec-16 that above 2200 MET-minutes/week there is increased cancer risk which outweighs the cardiovascular benefits. Now admittedly it is a very weak correlation, and MET measurement is imprecise, but I think it is safe to say that there are risks associated with over-exercising and while being in shape is good, being a ultra-marathon runner or other extreme fitness enthusiast is unhealthy long-term, and the risks start sooner than you might expect. (2200 MET-minutes/week is about 3 hours of 8mph running per week)