As a survivor of a recent heart attack, I would like to make a rather surprising suggestion. If you live in big city, get a job as a bicycle courier. If near a college campus, get a job as a bicycle pizza delivery person. Get someone to pay you for getting healthy exercise. Then you can spend your spare time on sedentary intellectual activities without damaging your health.

If you are like Mitchell_Porter, and wish to spend your spare time on serious math-like creativity, then you definitely need 4-6 hours of mindless physical activity in the middle of your wake cycle, with intense intellectual activity at the beginning and end of the day. No one can maintain peak intellectual productivity for long periods without some scheduled downtime.