He went on to break this time in Antwerp on 30 May 1969 by more than a minute, this time stood as the world best for nearly another 12 years.[nb 1] His personal best time of 2:08:33.6 is still a world class marathon time. Clayton represented Australia at the 1968 Summer Olympics in Mexico City, finishing in 7th place (2:27:23). Four years later he finished up in 13th place (2:19:49) in the same event.
^The International Association of Athletics Federations has published a progression of road racing world bests and records that were widely recognized prior to ratification and official acceptance by the IAAF. According to that progression, Clayton's 2:08:34 performance in Antwerp on 30 May 1969 was a world best at the time. Other road racing authorities, including the Association of Road Racing Statisticians, consider Clayton's performance to have occurred on a short course and recognize other athletes in the progression for world best in the marathon.