Most people work out when they can, so it might depend on where you can fit things in, but the ideal fasting and exercise setup might be to fast overnight — i.e., from 8 p.m. to 8 a.m. for a 12-hour fast, or 8 p.m. to 10 a.m. if you're going for a 14-hour fast — and exercise first thing in the morning, before you eat, ...