You might check out this site http://www.thelongridersguild.com/LRG.htm about people who actually do such long rides.
Another point to consider is that horses are grazing animals, and have evolved on a fairly continuous food supply moving through the gut. They don't do well on one or two big meals a day. Also for additional verisimilitude, you might want to put in details about hoof care &c. And remember, horses aren't sports cars!
PS: Another practical point is that on a long ride, it's rather nice to get off and walk with the horse occasionally, otherwise you can get pretty stiff. (Or even trot: I can keep up with my horse pretty well at a moderate trot, though I've never managed a canter :-))