My boy had colic surgery 5 years ago, followed by laminitis, so he had a long time off. He lives out though so has a base level of fitness from wandering around all day. When it came to start him off again I did just what people do when their horses have had a summer or winter off - started off in walk, riding about 40 mins, long reining for longer, and gradually increasing the duration and adding trot and canter. He never struggled at all.
Mind you we only hack - someone else might be able to advise you on starting schooling and/or jumping again if that is what you do.