Most horses don't get enough exercise. When most horses get exercise, they usually get too much. That's because in both cases they are dependent on us, and can't make their own determination.
In a natural state, a horse will walk, jog and occasionally lope as the mood strikes him. He'll travel about 30 miles in a 24 hour period. Of course there are plenty of stops for drinking, grazing and snoozing. In a natural state, a horse will get himself fit for the job he is doing, which is surviving, and that will be that.
In any captive state other than a planned training regimen, horses stand, walk in tiny circles, snooze, eat, drink, and travel only a few miles in a 24 hour period.
After five days in a 12 by 12 stall, thousands of horses are then asked to go trail riding, or roping, or team penning, or barrel racing, or to a show or just to zip around an arena for an hour. That borders on abuse.
So what kind and how much exercise should a horse have?
One renowned expert devotes a whole paragraph to exercise in his complete encyclopedia on horses. He says horses should exercise as much as possible on pasture. If no pasture is available, exercise mature animals for an hour or two a day under saddle or in harness. Thanks much, but they just doesn't get it.
Obviously different amounts of exercise are correct for different horses. Young horses, for example, don't need the same type of exercise as an older jumper.
A medical expert advises a horse's daily exercise should consist of enough work to make the horse's pulse, respiration and perspiration output increase to the point where at least one of the three is noticeable. That's not good enough either. If we follow that guide, all we know for sure is that exertion has taken place.
I think the answer lies in the dictionary. Exercise is defined as "active use to give practice and training, or to cause improvement."
A young horse in a round pen, or on a longe line is going to show improvement in gait, pace, stopping ability or just in paying attention about the same time he's had enough exercise to rid himself of all his excess energies. It won't be coincidental that his pulse and respiration are elevated, and that he's about to break a sweat on his neck. This is the place to stop. More exercise for this horse is going to be detrimental.
An older horse in training is going to need a few minutes just to warm up. So the rule of "walk the first mile out" is a good guide for starters. Then we can go to work on the performance lessons, past and present. Using the definition of exercise, we'll know the horse has had enough when he starts to show "improvement in his work." Now walk the last mile back. With the young horse or the older, conditioned horse, you'll be smart to never get them overheated mentally or physically with too much exercise. Horses, just as humans, learn best when they are fresh and feel good. There's another type of exercise horses need. Every horse should have some time alone, free, to roam. Turn him out in a paddock or a pasture, or even a training ring. Be sure he's got enough room to run, stop, turn, kick up his heels and get the kinks out.
Once a week or more often, you'll find the free exercise period will "improve" a horse's mind.
If you aren't going to turn him out, or longe him, or ride him, at least get him walked every day. Hand-walking a horse isn't all that much fun, so hot walkers are dandy. A young horse should have at least a half-hour on a walker if he gets no other exercise that day. A mature horse which is being worked four or five days a week will need at least an hour on a walker.
Now I agree, time doesn't have a true relationship to a horse and his present physical condition; I'm just trying to suggest the horse needs to move a good distance. Time on the walker may not be the best exercise, but it won't be too taxing.
I'll stick with the amount of needed exercise to be that which produces improvement in an individual horse's skills. Push the horse beyond the improvement state, and the horse's performance will suffer.
The key to exercise is knowing when enough is enough.