My suggestion will really only work if your daughter is in to this... but my friend had an overweight son with a speach impediment and got him a little mutt. He really loved dogs, and the puppy was a constant source of exercize. He didn't like sports because he didn't like talking to the other kids.
Again, if your daughter doesn't like dogs that probably won't help. Dogs (IMHO) are better than playmates or exercize gyms because of their unequal compassion and loyalty. The dog won't care if she looses weight, he just wants to have fun with her! If she can't keep up with him, he'll always come back to her side, full of unconditional support and encouragement.