This is a good point, I was test riding a few prospect horses a couple of weeks ago and I rode for about an hour two days in a row and I was feeling a little wore out to say the least. It kinda reminds me of a slip n slide when you were a kid, fun while your doing it but you hurt in places that you didn't know existed the next day.
If you got one to ride I would say in a weeks time you can get your legs pretty use to it and after a coulple weeks your good to go.
Matt