My opinion is that you body has a limited capacity to do anything. For example if you are weight training, you might improve year by year, but eventually you will hit a limit of what is humanly possible and won’t be able to make any gains.
Willpower is probably similar (but in a much shorter timespan). Willpower has to be a limited resource, since by doing different activities you consume energy and thereby have less energy available to do other things. The fact that you have less energy impacts your willpower.
However on the other hand, the human body is capable of much more than you think. That’s where the effect of the second wind comes in. At some points you are able to muster up your last amounts of energy and push through, right at the time you thought you were done.
I have been working on my own framework and categorization of cognitive biases. I would like to hear your comments (don’t be too harsh :) )…