"Lack of activity destroys the good condition of every human being, while physical activity saves it and preserves it" (Plato)
Instead of writing a post I am just going to link to an article in the British Journal of Pharmacology, in the article they postulate the idea that exercise can be considered as a drug. I urge you to read the article Exercise acts as a drug; the pharmacological benefits of exercise but these are the concluding remarks
"Exercise is so beneficial for health that it should be considered as a drug. As for any other drug, dosing is very important. Otherwise, unfavourable side effects may occur. Some of the favourable effects of exercise apply to the general population. Prominent amongst these are its role in prevention of many diseases and in the promotion of healthy longevity (see Figure 2). But exercise can also be considered as treatment of established diseases. These include commonly occurring conditions such as depression, diabetes or cardiovascular diseases."