In March 1939, a race was conducted from Nocona, Texas, to the San Francisco World's Fair. Though it was called a "Pony Express" the distance was only 1750 miles, but on the other hand only one contestant rode as an entry on two horses, alternately; the resting horse took part of his rest in an auto truck.
The winner came in under twenty-three days, which does not compare with the eight days the two hundred horses of the Pony Express used over 1980 miles from St. Joseph to Sacramento with forty riders.
This gives some seventy-six miles a day for twenty-three days two horses, one rider. The horses used in the test were simply good ranch-horses. The winning rider, Davidson, must have been out of Piano Wire by Chromium Plate. The weather conditions ranged from freezing to 108 degrees Fahrenheit in various parts of the ride.
The ostensible notion was to prove that good range-horses were good, without any particular pedigrees, ribbons, or anything of the sort. This was proved.
(Source: Strong, Horses and Americans)