Some interesting tidbits on horse endurance in this article:
http://www.si.com/vault/article/maga...4149/index.htm
A "Berlin-to-Vienna endurance race" of 360 miles completed in 71 hours an 20 minutes. That's an average of 5 mph. A count won the race, but the horse died at the end. of some 230 entrants, 30 horses died. Which suggests they really pushed it.
The 1,000 mile race from Chadron to Chicago had the following rules: "Only western cow ponies could be used, and only two horses to a man, with ordinary 35-pound range saddles, the saddle and rider to weigh not less than 150 pounds." And At the end: "[The ride] covered the thousand miles (set later at 1,040) in 13 days and 16 hours, the last 130-plus in 24 hours and the last 80 miles in nine hours and a half. Humane society veterinarians examined [his horse] Poison and pronounced him in fine shape. "Western range horses," cried Buffalo Bill, "are the hardiest and best horses for cavalry use on the face of the earth!"