The Namibian desert won't be kind but two Canberra women are excited about running 250km in seven days for the Sahara Desert Race on April 30.

Running the ultramarathon, Charlotte Lynch, 24, and Nicola Roth, 23, hoped to raise $10,000 for Rare Cancers Australia, a charity raising awareness of rare cancers.

Ms Lynch's brother Nicholas died at 21 four years ago after a rare cancer diagnosis, desmoplastic small-round-cell sarcoma.

"There were only a few people across the world diagnosed with it, so there's not much money," Ms Lynch said.

The Lynchs united around Nicholas, the experience bringing them closer, but Ms Lynch said it wasn't the only reason for participating, as Ms Ross originally found about the race and had been close to Nick too.

"When we decided to do it, we decided we should do it for a purpose," Ms Roth said.

"I think that played a big part about why we chose rare cancers specifically."

Of course, they had plenty to consider: did they have enough time to train? Do you take regular sneakers to the desert? Could they handle the heat?

Moved to the Namibian desert after security concerns in the Egyptian Sahara, competitors complete about 40km a day in 35C maximums and 0C lows, past shipwrecks in the sand with organisers providing only medical care, water and campsites.

"We've always been pretty adventurous, I think it's just another way of challenging ourselves," Ms Lynch said.

The pair were training on Sunday morning, part of their regime of 70-100km a week, the goal being 100-150km per week, as well as hill training and leg strengthening.

A lot of what they'd heard from other runners was around the importance of being mentally prepared, but they'd have each other and joked most of their long runs ended in silence.

"I know what [cancer patients] go through, so I'm going to run this race, this is just nothing compared to what those people go through and also peoples families," Ms Lynch said.

Most of the racers are men, an even smaller percentage are under 29, so as two young women, the duo will be a rarity, especially considering they'd never run marathons.

Their families and friends were sceptical at first, worried they hadn't thought it through but their commitment has finally won everyone over.

Kalia Chan, 24, a friend of the girls, was having coffee with them after training on Sunday.

"I think they're crazy. But it's actually so good, I'm actually so proud of them," said Ms Chan.

People interested in donating can visit 10kfor250km.com. The travel is self-funded and 100 per cent of donations will go to charity.

Clarification: In an earlier version of the story, Nicholas Lynch was incorrectly named as Ms Roth's brother. He was the brother of Charlotte Lynch.