He began running at age 55 to combat mild hypertension. On 30 September 2001, at age 76, he set a world age record when he ran a 3:22:59 marathon in the Portland Marathon. This was after a comeback following an accident that may have ended his running career. On 2 April 2005, at age 80, he ran the 15k in 59:00. In 1996, at age 71, he barely missed becoming the first runner over age 70 to break 3 hours in the marathon, as he posted a 3:00:58 at the 1996 Twin Cities Marathon. This is still the fastest ever marathon by a 71-year-old, although the Canadian runner Ed Whitlock subsequently ran a marathon in under 3 hours at age 72, 73 and 74. At age 78 Keston set a world age record for 12-kilometers in the Bloomsday Run.