Pony Express service between St. Joseph, Missouri, and Sacramento, California, demanded exceptional speed and stamina from both horses and riders to carry mail in 10 days or less. Thomas O. King remembered that only the best riders, physically able to endure day and night travel in all weather and other dangers, could do the work. Eighty tough, experienced youths were hired to ride either active western mustangs or the finest American racing horses available. Traveling to Salt Lake City in 1860, Sir Richard Burton observed that riders covered 100 miles at a time at about eight miles per hour, changing horses four times, and returned to their stations the next day. Riders had to remain in the saddle over grueling distances, with or without relief, and Nick Wilson recalled that many could not stand the long, fast riding at first, but hardened after about two weeks; once started, they were not to turn back for any reason until the mail was delivered at the next station, and they had to be ready to leave again at half a minute's notice in any conditions. Alexander Majors later wrote that their service demanded constant vigilance, bravery, agility, strong wills, and remarkable powers of endurance.