She's so cute! LOVE her!
Her back looks really great! You'll want to preserve that. You can help her build back support by lunging or in-hand work. Have her lower the head and neck, and step under with the hind legs to lift the back. Do that while walking around in an easy circle, about 20m. 10 laps in each direction is a good warmup exercise, but you can do more.
Also, might want to consider supporting give her joints a boost. Cosequin original (or similar product), Lubrisyn HA+ (hyaluronic acid with MSM), and Manna Pro Senior Weight Accelerator for Vitamin C and E (2 scoops). Contains no sugars/starch. If you're concerned about colic, introduce to her diet slowly and make sure the mixture is 'wet'. Lubrisyn is liquid, and you can add a little water to create a wet feed that she can lick it up. If she has an issue with eating it, you can mix 1 cup of your pelleted feed with water and add it to the mix. Of course, discussing this with your vet couldn't hurt - some vets will answer questions over the phone.
No diet, no hoof. No hoof, no horse. No horse is not an option!