Oh, goodness, PackBrewer - I really feel for you, and for your daughter. I can say I've been there (as the daughter) and nothing made me feel more like eating than being told to lose weight. I really would advise you not to even mention it. She'll know well enough what you're thinking, and even the suggestion of a walk might bring on that defiant "it's my body" reaction. I strongly endorse what Cate says, about "don't make a fuss".
I know topic is few months old, but I still feel like replying to it.
I would stick with aiminglow here. I was also fat teen, I think I was hitting 220 lbs when I was 15. And I can say what was helpful, and what wasn't in my case.
Worst thing were, as aiminglow said, even mentioning my weight. These years are pretty mentally fucked up, especially for girls. I must admit somehow I could handle my mom's gentle comments. But when my dad mentioned my weight for the first time I was devastated. I couldn't believe what I'm hearing. He snapped unfortunatelly, because probably some members of family were complaining of his daughter's weight, so he redirected his frustration directly towards me. It was terrible, he was screaming, I was crying. Communication with male authoritive figure is crucial for girls of that age, and it's very bad when stuff like that are even mentioned, even with good intentions. I believe things would be easier if your girl had a mother, but it's how it is.
You are alone with her, and you are worried for her health. That's good start. I like how you aware that you gave her bad example while you were fat. That's probably good place to start developing emphatetic approach.
I can continue my story and tell you what helped me. I went to the regular doctor appointment, and of course, the doctor suggested weight loss. My mom was with me all the time, she wasn't judging me at all, she listened to doctor's words, she smiled, and said thank you for your suggeston. We went out and she gently mentioned that we should make some food plan - together. She will be here, she will take care of my meals and she will help me. I felt safe and supported, I had doctor's opinion which was a harsh fact, but on the other hand I had mother who was willing to help me.
I have gave up bread and sweets immediately, I started doing dance classes 3x a week, and being more active, and next year in school I was down to 190 as I remember.
The most important thing was that she wasn't judgmental at all, and that she SUPPORTED me, in best way possible, she never commented size of my butt, and she took care to make lighter meals. I needed someone to be with me trough all that, and I had that. The dance fitness part was something I had to do on my own, but I met other friends there so they kept me motivated and regular on the classes. And mum was always giving me enough money to pay for classes, I just needed to show up there.
Maybe this story will help you and your daughter. In conclusion, the worst thing you can do is to even MENTION it without actually trying to help (like my dad did). It would be great if you could cook meals for you and her together. If you eat healthy, make her a breakfast as well. Eliminate bread and sweets out of the house. Spend some time on hiking or similar activity, but don't focus on weight. Go hiking but talk about all other stuff.
And yeah, consider going to medical checkup together with her, probably doctor would mention her weight, and you will be there to support her and talk about solving that problem - together, like my mum did. But don't judge, sit, listen , and talk gently. Tell her that she is beautiful and precious to you in every size, but that health has nothing to do with beauty and size. Separate those things, make her learn the difference. You have to be here for her, she shouldn't do that all alone.