I know you intended your comment to be a little tongue-in-cheek, but it is actual energy, measured in Joules, we're talking about. Exerting willpower drains blood glucose levels.

Some doubt has been cast on that theory (Googling /willpower glucose/ turns up various papers for and against), but besides that, someone reporting sensations is not reporting the physiological causes of those sensations, even if they have a belief about what those causes are.

while very few people enjoy activities that force them to exert willpower

There's an annual 100 mile bicycle ride at my home town that gets above 3000 participants every year. There are 50 and 25 mile options, and perhaps only a minority do the full 100, but it's still a sizable number.

Anything that one is serious about wanting to do, one will exert as great an effort as required. "Having to exert willpower" sounds more like not actually wanting to do whatever it is but grinding on with it anyway. It's the activity that's unenjoyed, rather than the effort.