Jim Brown
Jim Brown played running back for, appropriately, the Cleveland Browns for nine seasons from 1957 to 1965, during which time he led the NFL in rushing yards for all but one year and helped the Browns win the 1964 Super Bowl. He obviously played at a high level throughout his career and is the only NFL running back to average more than 100 yards per game, but he had other interests outside of football and retired at 29 years old to focus on an acting career.