I would have a vet look at her and tell you exactly what kind of work she can do.
Next, make sure she's getting only grass type hay, no hard feed or chaff. Restrict her hay to 2% of her IDEAL body weight a day, split into 3 feedings. Give her a stall toy and daily turnout in the arena or a paddock with not so much grass.
I would also give her a handful of grain or chaff with a Magnesium supplement. Magnesium helps some horses better utilize the sugars in the diet, which can help reduce obesity. Talk to your local feed store to see what they can get for you.
For exercise, if you are able, hand walk her around the property and on the trail. Switch sides that you're leading her on often, and work on basic ground manners while you're out there.
If you have access to a trainer, get some help in learning to lunge and teaching her how. Lunge her just 10-15 minutes at the walk and trot, making sure she doesn't get over-tired.