Although racing over rough terrain for 100 kilometers with a seven-kilogram backpack containing food, water and emergency supplies might not seem like a holiday, more and more people are trading in five-star resorts for this more extreme kind of adventure.

Getting off the sun lounger and into the hills is not cheap -- race entry fees alone can run up into thousands of US dollars -- but there's a growing group of high net-worth individuals who prefer their vacations to include a trot up a mountain or two.