I think if a person can just force themselves to do something about a dozen or so times it becomes habit, and then it's not so hard to keep at it. But like wise if you get those good habits going, then you start sleeping in a few days or take a break from eating well or exercising, those become habit too and they can be hard to break.
It would be so nice if someone could invent a pill or potion that would make us somehow crave the good habits, but hate the bad habits. Maybe crave and hate are not the right words. You know how if you've never worn glasses before and you start, it feels weird? But then after a while if you forget to put them on, you feel naked? We just need to get to the point where we feel naked (or that something vital is missing) when ever we don't exercise. But getting there can be SO HARD some days!