walking walking walking and more walking. Walking is far less stress on the horses joints while getting the heart rate up and blood pumping. Walking up and down hills is best!
Think of it as if you were getting back into shape after a long time off. You wouldn't go straight out and run 5km. You would walk for a few sessions first to get your heart rate up, blood pumping and muscles adjusted to the increased strain.
Make sure that the work is marching and active, you want him to be 'power walking'. I like to start with 20mins each day for a week, then start to incorporate a few minutes of trotting into each walk session. Rather than doing one trot and then finishing up, I prefer to walk for 5 minutes, trot for 2 minutes, walk for 5 mins, trot for 3 mins and so on. It allows the heart to pump at a certain rate at the walk, then it has to increase it's rate into trot, then it can go back to a slightly lesser rate at walk, then back up again.
In humans that type of training is said to be the most efficient for burning calories and increasing fitness without too much strain on your body as the heart rate is constantly changing rather than remaining at a steady rhythm.
After a month of walk/trot work, I then start to add some canter into the equation. Just light pony club style canter, lots of big circles and straight lines to get him nice and loose and balanced.
Plenty of trot-canter transitions will start to build balance and suppleness, as well as promoting the increase of muscle over his topline.