Exercise is very important. I wouldn't recommend looking at it as something that is optional. It's funny you mention willpower, as there is a book called "The Willpower Instinct" and the author claims that willpower is indeed finite and exercise is one of the best things that can lead to a greater amount of willpower over time. So exercise may be a short term willpower cost but if this author is correct, exercise would lead to a net gain in willpower over the long term.