When you are in that situation, there are two ways you can go - either hit the point where you can't stand yourself like that anymore, or you just keep cruising through life as you are now. Sometimes you need to come across that one thing that gets you excited in order to show you that you can't achieve that thing w/o toss all your bad habits Sometimes you don't.
For me, I always went into the gym. I used to mess around w/ the Universal machine at the YMCA when my dad & i used to play racquetball there. I found I could really kick on the little leg press machine & it was cool. And back then you didn't see girls lifting weights. So it was my unique thing & I coudl do it sort of decently. When I went away to college I was always a nervous wreck because of the pressure of being away from home, having a whacked out roommate and having a shitload of homework. So I started swimming laps at the school aquatic center. In the winter when it was too cold to go there, I found the weight room. Been there ever since.
Going to the gym is the one thing I can do anywhere I'm at-- when I travel, move, whatever. I can always locate a couple gym rats who will spot me or just yak about whatever. Since I moved to my new place, in a new town, at a job that hasn't turned out to be quite what they told me when I interviewed, the gym has been my one focal point. I spent 2 years being depressed about several things, getting fat, slouching & sitting in front of the tv alot and managing to screw up my lower back. Its all almost fixed now.
For you, when you experience something that you can't have, do, or achieve because of the way you live now - that is when you will find the 'willpower' to go do what you need to do. And one good thing perpetuates another.