The Otillo Swimrun world championship, held every September in Sweden is the granddaddy of Swimrun events and was labelled by The Guardian UK as the toughest one day event in the world. The team at Otillo literally invented the sport of swimrun and this race is what turned it into one of the fastest growing endurance sports out there. Race stats and logistics: Trail running: 65km Open water swimming: 10km Islands to cross: 26 Amount of transitions: 50 Longest run: 19.7km Long