It's known as the toughest cycle race in the world and in two weeks Kiwi Craig Harper will attempt the gruelling Race Across America.

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The Blenheim cyclist will be crossing 12 states along a 4,828km route from America’s west to east coasts. And here's the catch - he has just 12 days to complete the journey.

"It's pushing the boundaries really beyond any other cycle race," he says.

The epic journey begins at Oceanside, California and takes riders through the Sonoran Desert of Arizona, Monument Valley in Utah, high over the Rockies of Colorado and down onto the seemingly endless Great Plains of Kansas.

And then it's into Indiana and Ohio, this week's deadly twisters a reminder of the potential risks during the tornado season.

The next challenge the Appalachian mountains of West Virginia and Pennsylvania before the finish in Annapolis, Maryland. By this time he'll have climbed enough hills to take him to the edge of earth's stratosphere.

Harper flies out of the country this evening, taking with him some handy endurance experience. The builder and athlete holds the world record for biking the length of New Zealand in less than four and a half days.

Since then, he’s been preparing for America’s extremes.

"Heat and altitude are big,big factors," says Harper.

"Probably the reasons why over half the field DNF (Did Not Finish) each year".

He’s spent time training at the University of Waikato’s Adams Centre for High Performance - a practise desert without the scenery.

"So I was doing 24 hours in the heat chamber at 40 degrees and 20 per cent humidity on the trainer bike. That was brutal yeah, really tough."

Building mental strength's been more important than muscle. And his support crew of nine will help keep the mind off the k's as he tries to grind out 22 hour days in the saddle.

Harper has purposely stayed away from declaring a certain finish time, "purely to not place limitations on myself". But he wants to be competitive at the front end of the field.

"There's some guys I'm up against that have done the race eight or nine times so there's huge amount of experience there. But in saying that, there've been rookies before that have won that race, so it's not impossible."