Around 90 runners will set out on a three-day 131-mile (211km) ultra-marathon around the coast of Anglesey on Friday.
Organisers say they have run the course - the equivalent of five marathons - along the Anglesey Coastal Path themselves to test it out.
The event has been called the 'Ring O' Fire' because of the challenges.
Competitors from Australia, USA, Poland, Italy, Wales, Scotland and England are taking part.
"It was very difficult because of all the ups and downs, the cliff paths and the sandy beach when you reach Newborough," said Quentin Brooksbank, one of the organisers, who is originally from Holyhead.
Participants start and finish at the Breakwater country park in Holyhead.
Joint organiser James Bingham, who is also from Anglesey, added the event would allow runners to challenge their limits.
"Ring O' Fire should not be underestimated. The 131-mile distance and 13,695 feet of vertical ascent make this a mammoth undertaking for even the most hardy of runners," he said.