I don't think that would work very well. The problem is the bad health effects of inactivity usually don't really kick in and effect morbidity until after the reproductive years.
What does that even mean? Its not like evolution did some magics that we become better with exercise. Our bodies are trained by actual physical exertions, which are not possible if one is sedentary.
Humans generally are more valued for doing desk jobs vs physical jobs today. You can survive doing physical labor, but it will be a low paid job. The jobs where you sit behind a desk and your only exercise is pushing a keyboard/mouse are much higher paying, and more stable. As such it is an evolutionary advantage to sedentary today: you are more likely to be able to afford food, and since you are not active you need less of it.
The above might sound good, it won't work that way. Exercise hard or be a couch potato - either way your DNA is not a dead end. Either way you get enough food for your and your kids. You will have more luxuries from the sedentary life, but those are not things evolution cares about.
There are other shortcuts. Bio engineering is promising. There are lots of other advances in medicine. Any of them could solve the problems of sedentary life in the future. Or maybe not, we don't know what the limits of each is.
