In their natural state, horses move over many miles of territory and get plenty of exercise. A savvy handler takes careful note of the instinctive way horses act when left alone without human intervention and then allows his animals to come as close to this state as possible. It's important to mention, however, that through classical conditioned response training, some horses do feel safest in a stall and prefer to spend a great deal of time in their own space, for this is how they've been raised. Generally speaking, a horse is a curvy creature of freedom. He must have access to open pasture to stretch his long legs and to snort at the wind.
Specific disciplines demand different kinds of conditioning.
What is required of a racehorse, for example, differs from what's required of a dressage horse. Specific training programs are far beyond the scope of this information, and the best advice to anyone who wants to get involved with a horse-related activity, be it eventing, dressage, or competitive trail riding is to seek the counsel of first your veterinarian, and next a professional trainer. Ask friends and conduct brief interviews, but it is imperative to work with an expert at training. The average person who rides a horse should think of the animal as an athlete in soft, shiny hide. Always work within the horse's physical limitations, as failure to do so can be disastrous.