A while back when I quit smoking (ended up taking it up again, though, cause I got a smoker girlfriend, then kept smoking when we broke up, and still do) I sort of did a whole lifestyle change. And along with that came eating healthier, and exercising/working out.
I found that after working out, I'd realize exactly how winded I was, and how much smoking was holding me back. I just slowly cut them down to a point where I was only a social smoker (when drinking with friends, pretty much always at bars)
But you'll definitely have some withdrawls, but the best thing about hitting the gym is that feeling when you're walking out the door after a workout. You look back and always say to yourself "That was the best way in the entire world that I could have spent that last hour of my life"
Another thing to look forward to by being healthy all over is getting healthy friends. Less peer pressure.
So put some cocoa oil on those stretch marks, you fatass, and get to the gym.