Try the book, "Going The Distance"
Maybe see if someone can lunge her for you if you can't ride her or get a friend or instructor to ride her? Also maybe put her in a big paddock with other horses so she can gallop on her own and move about?
Try and do lots of gymnastics and go on lots of hacks
Cardio rides. They are specifically meant to strengthen your horse's cardiovascular system, and in turn will improve fitness. Cardio rides should be very forward and fast paced, with lots of extension, and big circles, and should mostly be ridden in the trot and canter, as well as contain non progressive transitions
Try a different feed. Sometimes if you switch feed to like a diet food it will help them lose weight. Where I work we have 3 horses in what we call that fat kid club and they're all on the diet food for weight control. (Sorry I don't know the name of the feed, but I could figure it out). You could also put a muzzle on him/her while on grass. Just to slow your horse down on grass but not cut back on grass time any more.