On 13 October 2007 (14 October UTC), Wellington won the world championship Ironman triathlon title at Kailua-Kona, Hawaii, she finished in 9:08:45, five minutes ahead of Samantha McGlone, running the marathon leg in 2:59:58, the second-fastest time recorded by a woman on the Hawaii course. Her victory was described as the "biggest upset in Ironman Hawaii history", "a remarkable feat, deemed to be a near impossible task for any athlete racing as a rookie at their first Ironman World Championships" and "one of the biggest shocks in the sport's history." At the Ironman Australia Triathlon in April 2008, her first Ironman since Hawaii, Wellington again won by a margin of five minutes, finishing 9th overall. Her marathon time of 3:01:53 was beaten by only two men. Only twenty days later, she attempted her first World Cup race, the Tongyeong BG Triathlon in Korea, but could only finish in 22nd place. She wrote, "If I judged every day by whether I win or lose, yesterday would be considered a 'bad day' [...] But I need to have these days - because the 'defeats' expose my weaknesses, and enable me to grow, learn and develop as an athlete. On 12 July 2009 at the Quelle Challenge Roth, Wellington set a new record for ironman-distance triathlon races of 8:31:59, beating Yvonne van Vlerken's record set a year earlier over the same course by 13 minutes and 49 seconds. Rebekah Keat, who finished second, 7 minutes 25 seconds behind Wellington, also beat van Vlerken's time. In October, Wellington won the world championship for the third time, setting a new course record of 8:54:02, beating Paula Newby-Fraser's record of 8:55:28 which had stood since 1992. She finished 19 mins 57 secs ahead of second-place Mirinda Carfrae, the 2007 Ironman 70.3 World Champion who, in her first Ironman race, ran a marathon time of 2:56:51, fifty-three seconds faster than Wellington's record of the previous year. Wellington's victory was described as "stunning" and "even more dominant" than usual. Only 22 men were faster than Wellington.

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