Competition between footmen gave way during the second half of the 18th century to men racing against time over long distances. 'Pedestrians' (as the walkers were called) could win a very handsome fee for walking dozens -- or even hundreds -- of miles within a proscribed time. Side bets were, of course, very welcome.One of the more popular goals involved covering at least 100 miles in less than 24 hours. Those meeting this goal were (and still are) called Centurions.
(quoted by Phil Howell in A Brief History of Racewalking in the United States, North American Racewalking Foundation)