You could say that. Discipline is how the habit is created and kept. The first thing I tell people is that exercise isn't easy. It's not supposed to be. Outside of all the health benefits, one of the biggest benefits of exercise is the mental growth of learning to accept being uncomfortable.
EDIT
I'll also add that most people who are disciplined don't really think about it that way. They just do what they need to do.
> one of the biggest benefits of exercise is the mental growth of learning to accept being uncomfortable.
I claim the same about hunger (within bounds of moderation etc etc). It's simply uncomfortable, but it's a discomfort that you can adapt to and live with. My general view is that both hunger and exercise work as ways to moderate weight but they rely on the exact same capacity for discomfort (and enduring that requires either necessity or motivation/discipline etc)
EDIT
I'll also add that most people who are disciplined don't really think about it that way. They just do what they need to do.