Here is an interesting article on how making decisions costs you energy and willpower. If you make a lot of decisions at the same time, like choosing among 56 interior colors for your new car, the theory says you will run out of energy to make other choices. Willpower turns out to be a choice, and a potentially difficult one at that; resisting that piece of cake requires a lot of effort. People that work in jobs filled with a constant stream of difficult decisions come home exhausted and don't have enough energy left over to hold out. Not really fair that glucose restores mental energy and helps willpower. Once you've eaten that cake it's too late!
Getting very hungry reduces willpower as well. I know that I can get short-tempered and snappy when I'm hungry. Grandma used to say "you'll feel better after you eat" and she sure was right. I act nicer, too.
Habit might be a saving grace in this regard. Forming the habit of running after work every day can make exercise the non-decision thing to do. Unhealthy choices would require more effort and happen less. But it takes time and willpower to gain good habits and that means a large reserve of mental energy. Perhaps using peer pressure and social obligation like having a running partner would be a more efficient way to build habits. Definitely organizing your life to conserve willpower sounds like a good idea.
