If he's that age and been out of work for 5 years, PLEASE take it easy. He does not need hard work yet. It's not only the heavy breathing you need to take into consideration - working an unfit horse hard straight out will enormously increase the risk of injury. It takes time to condition the horse's body to working again.
Walking walking and more walking is the best place to start. Give him at least a few weeks of hand walking, up and down hills, along the road etc. Lots of ground work, yielding the quarters and shoulders, backing up etc will all help to strengthen and develop flexibility in his joints.
Horses are such giving creatures and it is all too easy to push them beyond a point that injury can occur. It is like asking a 70 year old, obese man who has sat on the couch and eaten junk food the the last 5 years, to jump up, put on some running shoes and run a marathon, or sprint 400m.The man would probably have a heart attack or do some pretty nasty damage to his soft tissue.
Golden rule - take it easy! Step by step, lots of walking for a few weeks, then start trotting etc. Keep the lunge work and circle work limited, straight lines are your best friends at this stage as small circles will encourage strain injuries.