Here are some ideas to help her get in better shape - I am not sure if they will be of any use to you at all, but they are things I wish my boyfriend would do for me.

- Don't bring any junk food in the house. If she brings in junk food, ask her to stop. Tell her that YOU don't want to eat it.

- Eat healthy meals together off of small plates.

- Tell her you want to a try a vegan/low-carb/whatever diet. Something you both can live with.

- Pick up a hobby involving exercise that you do together. Walking, hiking, swimming, biking, kayaking.

For all of these things, really highlight the benefits you will get. Don't point out that you want to do this stuff because she needs to be in better shape.

She's probably picked up on the fact that you think she's fat and unattractive. That's a desire killer. Could you convince her that you think she's beautiful and desirable?