Giants from the world of athletics, rugby and super biking compete against some of the toughest local champions and each other in four challenges across the globe, each one harder than the last.

Starting in the heart of Mid Wales, it's the first leg of their challenge and a chance for the sportsmen to get to know each other. Donovan Bailey, Gareth Thomas, Iwan Thomas and James Toseland will be taking on the notorious fell run, Man Versus Horse, 21 miles of endurance running over some of the most hostile terrain in the UK, racing against horse and rider.

As they meet in the small Welsh village of Llanwrtyd Wells at the pub where the run originates from, the scale of the challenge starts to dawn on them. The race is a real test of stamina as the runners take on punishing hills, cut through forests and wade through freezing water in the exposed countryside.

Hailing from different sporting disciplines and with varying degrees of fitness levels, the foursome will have to do some serious training over the next four weeks if they want to complete the course. But as three of the group get stuck in with some hill practice with former race champion Dic Evans, it's evident that one of them is starting to struggle. Former sprinter Donovan Bailey is not used to running long distances, and he soon finds himself lagging behind. He misses the comforts of home, and his enthusiasm for the challenge is waning.

In order to conquer 21 miles, they will need to push their bodies to the limit. Who has got what it takes to triumph in the first test? Show less