You may have thought of this already but you're out of shape because of your eating disorder. You're heart is working overtime so you get tired quicker. I know i have a hard time exercising anymore. I used to love it. I ran X amount of miles everyday and now i have to force myself to run one. Not because i dont want to but because i can't. I get dizzy. Soccer also takes a lot out of you. Its probably one of the most active sports.

I guess wut i'm trying to say is dont be too hard on yourself because you end up putting yourself in a worse place. If you didn't worry about being in shape as much, you wouldn't get so down on yourself and end up doing worse to your body.