If you know your horse has a tendency to turn a leisurely trail ride into an all-out race, set the situation up at home so that you can safely correct him before taking him out on the trail. Horses that grab the bit and charge ahead of the group are not only dangerous to themselves, but to everyone else on the trail.
Find a controlled environment, such as an arena or a large pasture, and enlist the help of a friend on horseback. Start at the walk and ride side by side, about 15 feet apart. Keep both horses on a loose rein and dare them to race ahead. If either horse speeds up, immediately pick up on one rein and turn the horses in toward one another and ride off in the opposite direction. When you turn your horse, do so with urgency so that he has to hustle his feet. If you just let him turn lazily, he’s not going to connect racing ahead with stopping his forward motion and having to redirect his energy.
Once the horses are turned, walk off in the new direction so that the horses are side by side again. Be sure to put your horse back on a loose rein and dare him to make a mistake again. In order for the horse to learn not to race ahead, he has to commit to the mistake. If you try to babysit him and keep him from speeding up, he’ll never get any better and you’ll always have to watch over him.
With repetition, the horse will realize that when another horse comes up beside him, it’s not a race, and he’d better keep his attention on you because at any second you might change directions and go back the other way. And if he does speed up, he’ll quickly realize that it is pointless because you’ll make him turn and go the other way. Initially, the hotter and more nervous your horse is, the shorter the distance will be between turns. Eventually, he’ll be able to walk next to the other horse on a loose rein without ever speeding up.
When the horse can do the exercise at the walk, move up to the trot. You’ll find that the faster the horse’s feet move, the more of a tendency he’ll have to race the other horse. Anytime he speeds ahead, pick up on one rein and turn him in toward the other horse. Then practice the same thing at a lope. It shouldn’t matter what gait the horse is in, he should remain at the speed you set him at and not race ahead.