My 6 y/o OTTB recently underwent epiglottis surgery (nothing major) but he was put out of work for 6 weeks. About a month ago I started walking, gradually built up trot work, and now I am doing some canter and trotting very small x-rails. I was hoping to be able to get him back into shape for Novice level eventing and my first event was supposed to be in June. However, he has lost most of his muscle mass and it seems like he is taking a while to build it back up. His safety is my top priority… what would you suggest I do to try to get him into shape quicker? Thanks 
How much overall work is he getting - are you riding 5-6 days a week? Can you tack on 30 mins walk outside the arena after every ride? Can you add basic sets in once a week? For a novice horse, you could shoot for 3 5 minute trots with 2 min walk breaks, followed by 3 3 minute canters with 2 min walk breaks.
I always try to go on a walk in the fields after every ride. If course I have fitness days with trot and canter sets as well. I also don’t give my horses days off during show season, even if I just get on and walk for twenty minutes.
Lots of walking is one of the best things you can do for them.
[QUOTE=asterix;8129413]
How much overall work is he getting - are you riding 5-6 days a week? Can you tack on 30 mins walk outside the arena after every ride? Can you add basic sets in once a week? For a novice horse, you could shoot for 3 5 minute trots with 2 min walk breaks, followed by 3 3 minute canters with 2 min walk breaks.[/QUOTE]
I work him 6 days a week. He is trotting about 20 min total and cantering 5-10 min spread out. I’ll definitely add more walking time 
Do you have hills where you are?
Lots of walking - with added hills is one of the best way to “leg up” a horse. Hills will start adding top line and booty has he has to work up them.
Once he starts adding muscle, and can walk hills without blowing - start adding trot work, and trot hills.
