Perhaps better known for her fashion sense than any athletic prowess, Pippa Middleton may be set to change the public’s perception of herself by taking on one of the world’s toughest endurance races.

Middleton, whose sister Kate is the wife of Prince William – second in line to the British throne, is one of 240 competitors taking part in the gruelling Ötillö (Island to Island) swim run race in the Stockholm archipelago on Monday.

“I’m inspired by the scenery, the sense of adventure and sheer distance that’s involved – running 65km in a wetsuit and swimming 10km in trainers with hand paddles,” she said in an e-mail as she completed her final preparations for the race.

“It’s relentless: almost three ironman swims and one-and-a-half marathons. Much of the running is trail running on uneven ground, hopping over rocks on the shore line, dipping in and out of bays.”

Celebrating its 10th anniversary, Ötillö started out as a barroom bet between four friends, and inadvertently became the fast-growing sport of swim run, combining trail running and open water swimming.

Covering 75 kilometres, the Ötillö race now boasts the title of swim run world championship, with qualifying races held in venues across Europe.Competing in teams of two, 240 athletes start the race, with thousands more on a waiting list.

Having undertaken the more traditional marathons, triathlons and cross-country ski races, Middleton jumped at the chance of a new challenge.

“It’s an unusual and unique event, not for the faint-hearted, and I was taken by the idea of competing in the stunning surroundings. Ötillö has an edge to it that stands out above other endurance races.”

As if the challenge of the terrain was not enough, the race is also against the clock – there are a series of cut-offs throughout the course, with competitors who fall too far behind finding themselves disqualified on the spot.