Constant work and conditioning. If your horse is unfit don’t do long work outs. Good 20-30 minute rides with lots of transitions and lots of walking will help. Doing longer more intensive work outs will just tire the horse. Also pole work to keep it interesting. Oh and gallop sets once your horse is a little fitter. So for gallop sets do 2 minutes solid of canter work, forward, back, circles, flying changes, just no trotting and walking. Then walk 5 minutes. Then 2 minute gallop set again. This helps them open up their lungs and will help make them fit. Do this once a week.
Also remember that the more work the horse does you need to keep up with a proper diet as well. The more work a horse does the more food and nutrition it needs.
It's important not to over work the horse since they will likely be out of shape. There are someday a I can get a horse huffing and puffing at just a walk. What you need to do is get the horse to engage everything in its body by making it flex and bend and carry himself through its back and not get lazy. Put an over weight horse right it rigorous excursus can be hard in them especially their joints and they can become sore and unhappy. As the horse gets stronger it's energy level will start increasing the the little things like proper circles aren't as hard that's when's you increase and slowly increase. Remember slow and steady wins the race and results in a much happier horse that will enjoy working much more. Once you shed even a little bit of weight then it's time to kick into high gear.
Keep the horse in work do walk trot canter and don’t stop. make sure the horse is working hard on the horses work and don’t give the horse a day off. I did this with one of my horses and it helped a lot