Palmer wins world's longest horse race Lara Prior-Palmer has become the first Briton, first female rider and the youngest person ever to win the world's longest horse race. The 1000km Mongol Derby took riders across Mongolia. American Devan Horn crossed the finish line first but her horse later failed a vetinary inspection. "I can't really believe it," said Prior-Palmer, 19, who is the niece of British equestrian and Olympic silver medallist Lucinda Green. Prior-Palmer, from Hampshire, added: "I came into the first station last because my horse was so slow and I had to walk him in. I thought that would be the end of my Mongol Derby. I knew that there were 30 people and nearly all of those 30 wanted to win and I really just wanted to finish. If you compare my first few days to my last few days, I was going so much slower and suddenly I just got the hang of it and how to ride the horses and what to do to catch up with the rest. The race, which is in its fifth edition, was won in seven days and is a recreation of Genghis Khan's ancient postal system - a mammoth network of 25 horse stations across the Mongolian steppe. Riders change their semi-wild Mongolian horses at each station - approximately 40km apart - and stay with the local nomadic herding families that run the stations and provide the horses.