You exercise because it makes you feel better, and for most, it helps keep your weight at an optimal level. It’s also one of the best treatments for insomnia and reducing insulin resistance, as well as being a wonderful aid in the treatment of depression. So the reasons to exercise are many.
If you start slow, and most importantly, listen to your body, you shouldn’t run into the problem of exerting yourself excessively.
If you’re a serious athlete, however, you may want to reconsider how you train.