Being a horse, one would expect they could "sprint", ie. gallop, for a decent period of time. Mine (the first one I bought, at Whiterun stables), can only sprint for about as long as I can. That is, just several seconds. Then he goes back to cantering.
Is this normal? If so, what's the purpose of giving a horse (and apparently the hardiest of horses in Tamriel, or so I hear) so little fatigue?