My horse is a trail horse
Take it slow. Probably a max of 20 min a day, w/t and as much Canter and your horse can manage at the start. I recommend doing a half seat in the Canter even if you're not a jumper as it encourages their back to come up more easily and to move more freely as they rebuild fitness. Refrain from sitting trot until your horse hasn't restablished his core and top line. Doing lots of forward and back within gaits is like doing crunches for horses and help them engage their core and find swing in their back so it's an easy exercise for rebuilding strength and fitness! Hacking or track work is also awesome for overall fitness!