According to Wikipedia, Buffalo Bill while working for the Pony Express covered 322 miles in 21 hours, forty minutes using 21 horses, (which is just over 15 miles an hour). His relief rider had been killed and he made the round trip between Red Buttes Station and Rocky Ridge Station. Quite a feat!
Over the past two or three years, I dip into an Anthology by J. Frank Dobie called I'll Tell You a Tale, ever so often when time permits. In this anthology Mr. Dobie wrote a tale of one Francois Xavier Aubry entitled Little Aubry's Ride. This Aubry feller was a little guy, weighing not much more than he could eat in a day or two, say a hundred pounds give or take a little. Well, this Mister Aubry was a tough little nut who traveled between Independence, Missouri, and Santa Fe New Mexico, delivering freight. He bet a thousand dollars that he could make the ride in six days. It was over 800 miles that he had to ride. The total time it took him was five days and 16 hours, actual riding time he figured was four and a half days. They said his saddle was bloody when he he got off the horse at the end of the ride.
Now, that's some ridin'!
Well, there is much more to learn about both of these horsemen at Wikipedia.com and also the famous writer J Frank Dobie and his writings. Interesting men, interesting books.