I barrel race. I know where your coming from. Start out with small work. Warm up, like start out with a crap load of trotting. Then canter. Then trot and canter equal amounts, obviously long periods of time, like a half an hour for each gait, but both directions. I always work my horse to a sweat, and then sprint him once before cooling down. Breeze him out, find a stretch of land to totally sprint. Don't do it frequently. Mostly do the warming up of trotting and cantering and eventually galloping. Then do all of that consistently and then breeze him a week or two later. Getting my point? I would also put him on joint support, I do that for my horse and his knees are healing tremendously.
"Doing what you like is freedom, liking what you do is happiness."