A complete response would fill a book! And, of course, the answer will be different for every horse, depending on his level of fitness, your own level of fitness, your style of riding, the facilities available, your climate and more.
Before you embark on any fitness program, make sure that each of your horses is sound and ready to ride. Is he healthy? Is he shod or trimmed appropriately? Get clearance from your veterinarian if your horse is recovering from an injury.
Then start riding. Any equine fitness program will start slowly with walking and maybe some trotting, then gradually increase either distance or speed, but never both at the same time. So, yes, it requires "running them around." But that doesn't mean your horse can walk dully along, slouching underneath you. Get him moving and be sure he lifts his back while he marches purposefully forward. Do exercises at each gait that improve flexibility and strength. For example, practice leg yields, transitions and circles to help keep you and your horse interested while you also increase fitness.
Keep track of what you are doing by wearing a watch during each ride and keeping a journal of the work so you can be methodical in how you change it. In other words, make each ride count. Even the slow work can build muscle for your horse.
Each week, either make your rides a little longer or increase the time at the faster gaits. Think of yourself as a fitness coach, just like any personal trainer. You probably will have to encourage your out-of-shape horse to push and stretch just a little harder, but never so much that he may injure himself. Unless you are very fit from some other sport or are used to doing a lot of riding, a good program should make you work hard, too.
The specific times that you spend at each gait and the rate that you increase it will need to be customized to each horse. For comparison, here is a sample program that I gave to a client for a horse who was recovering from a very mild episode of laminitis. Before they began this schedule, the horse was sound, getting some turnout and was walking under saddle every day. He was well shod and the work was to be done primarily in a very nice sand ring---soft, but not too deep. This was intended for a riding schedule of four to six times per week.
Week 1: 30 minutes per ride with 5 minutes trotting
Week 2: 30 minutes per ride with 10 minutes trotting
Week 3: 40 minutes per ride with 15 minutes trotting
Week 4: 40 minutes per ride with 20 minutes trotting and 5 minutes cantering
Week 5: 40 minutes per ride with 20 minutes trotting and 10 minutes cantering
For many horses, this would not be the end of the program, but just the beginning. However, for some this would be too aggressive, and they would need to go even slower. A good way to monitor your horse's fitness level is by learning to take his pulse; then you can make notes of his heart rate before and after your workouts. After working, a horse's heart rate should return to normal within 15 minutes. If his pulse is still elevated after 45 minutes, then the workout was too much for him, and you'll need to scale back. Body soreness, pinned ears and other signs of pain or resistance may also point toward a horse who's working too hard.
You'll find a lot of books about conditioning horses for various disciplines. Be sure to find some that fit your needs. But nothing can beat having a good, experienced trainer right there to help you make wise judgment calls as you go along!
Melinda Freckleton, DVM
Haymarket Veterinary Service