The Long Walk by Richard Backman (Stephen King)

Richard Bachman, tells the tale of the contestants of a grueling walking competition where there can beonly one winner - the one who survives.
In the near future, when America has become a police state, 100 boys are selected to enter an annual contest where the winner will be awarded whatever he wants for the rest of his life. Among them is 16-year-old Ray Garraty, and he knows the rules - keep a steady walking pace of four miles per hour without stopping. Three warnings and you're out - permanently.