I saw this and just had to post a reply, it seems like all the advice is from guys...but what about the girl perspective of someone thats been in that situation...?
I myself have always been yeh admitting, overweight, but I've also been trying to do several things to beat that and start kickin Azz. I started to date this guy who was SUPER in shape, worked out everyday, cut, lean, and ate clean...and cldnt figure out wtf he was dating me, we had a great relationship, but after a few months he made a comment about my weight after a conversation of 'where our relationship was going'....he said, 'well I really love you, but seriously...i'm worried about your health, I know you're just a little overweight now, but what about when your 40? I mean, I want to live forever, and be healthy.'...of course I got incredibly pizzed bc it was kind of out of the blue and yeh, we had just got done having sex (PS, THATS NOT A GOOD TIME FOR THIS CONVERSATION)....I didn't talk to him for a bout a week, but in that time I really thought about everything...
He was right, and I had to take a look at myself, also looked at his point of view of how he was doing all these things to look hot awesome, and there I am, 'his fat gf' ...I felt bad. But I also should mention that even if he didn't bring that up, some one would have sooner or later, so I really couldn't be pissed at him. So what better resource then my already health concious bf, who would be there for me 24/7 and be not only my role model but my cheerleader!
We got a gym membership together and everyday after work we'd work out together, and he'd pack my lunches and cook me dinner, it was freaking great! Not only did I lose wieght, but I looked freaking hot, felt GREAT, AND did it healthy, not to mention our sex got freaking OUT OF CONTROL! It was awesome....even our relationship grew stronger.
So really, if you super love this girl and are looking at the big picture, sit down with her and have a serious talk, no doubt she's gonna be pissed, but if its for the long haul, its worth it for the both of you. If she's game, then you gotta be super supportive and make it fun and yeh be the cheerleader, but why wouldn't you be, your chick is now looking hot awesome! woo-hoo! If she's not into it, then there you go, and thats that, you can't force her, and from what I've read, this might not be the only problem in your relationship.
Good luck dude.