How long can a horse sustain a gallop? The distance a horse can maintain a gallop depends on their build and physical fitness. A well conditioned horse can easily maintain a gallop for a mile to a mile and a half. At two to two and a half miles most horses will feel fatigued. Lighter built horses (Arabians and Thoroughbreds) can maintain a gallop over longer distances than heavier horses (Draft or Quarter Horse type), and horses with longer strides can travel longer distances with less effort.
A horse is built to cover many miles in one day, but not at a gallop. A horse can cover more ground, faster, if kept consistently at a trot. While a horse may be exhausted after a three mile gallop, that same horse could trot, with a few walk breaks, 15 miles without extraordinary strain.
Most people assume the Pony Express riders galloped their entire route. In fact, the speed of a pony express rider averages out to 10 miles per hour- meaning they spent most of their time alternating between a trot (about 8-9 mph) and a canter (12-13mph). The Pony Express riders switched to fresh horses every 10-15 miles.