Motivation is more akin to an emotion than anything. Sometimes it's there, sometimes not. And you're not particularly in control of when you have it.

Discipline is something else entirely. Discipline is what you have when the "why" question becomes meaningless. You do it because it's what you do. Some say it takes time to build, I'm of the belief that it's a choice you make, both when you initially decide to commit and every day thereafter.