Hello. Thank you for asking your question. My name isXXXXX will try to help.
I would give her as much turn out as possible. If she is used to staying out 24/7, then that would be best.
Bring her back slowly. Ride her for 10 minutes, 4-5 times a week, for the first two weeks. Then add on 5 minutes every week until you are up to 45 minutes. I think 45 minutes will be enough for her per ride.
Do a lot of transitions, like walk, trot, halt, back up, lope. This will help build up her hind end muscles.
Make sure your saddle fits her. If you do not know how to determine a good saddle fit, then call the tacks stores, or ask around, or call the company who makes your saddle until you find an expert who can help you.
Make sure you get her teeth floated every 6-12 months.
Give her chondroprotective supplements and injections. The help her joints last longer. Cosequin is a good oral one. Adequan, and legend, are good injectables.
To keep her in shape, once she is fit, you need to ride her 4-5 times a week for 30-45 minutes. You need to do some walk, trot, and lope in each work out. Obviously just walking, for 45 minutes, would not keep her in shape.
I hope this helps.
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