I can see three ways of getting these in line. The obvious one is that if you do what you have to do in less time and with less expenditure of willpower, there is more time and energy left to challenge yourself.

The second one is what I do: Schedule challenges as a ritual. What I do is solve a projecteuler.net project every morning. Helps me stay sharp all day.

The third one is a bit meta: You can have a constant thought process going that examines your rituals and injects as much challenge as you can handle in the form of optimizations. This way you can keep improving your rituals, keep things interesting, and gradually free up more time for productivity. This is what I aspire to.