Chrissie Wellington had no previous history of being any good at any sport and then smashes the world's best in the Ironman after 6 months of training (?). Her coach was the controversial Australian and convicted child molester Brett Sutton who has a record of taking no-hopers to his (hard to find) training camps where he converts them into world beaters. Several of Sutton's athletes have been busted for PEDs or complained of his controlling nature (including making them take "special drinks").
Added to these factors is the almost complete lack of out of competition drug testing of Ironman athletes because it is not an Olympic sport.
In fact I'm surprised no one has actually exploded during an Ironman from excessive drug use.