Lucja Leonard will be running beside her best friend Marina Ranger, 25 from London, for the breast cancer charity Pink Ribbon Foundation.
The 37-year-old chose the cancer charity after three of her aunts and her mother in law were successfully treated for the disease.
The pair will head to the Netherlands next month to take on the gruelling Pieterpad, a 310 mile run from the North of Holland to the three land point near Maastricht.
They aim to complete the ultra marathon in just five days - and hope to raise more than £20,000 for charity.
Lucja, said: “We are two fun loving blondes seeking adventure through challenging our minds and bodies by running the length of The Netherlands in our pink Runderwear and raising much needed funds for Pink Ribbon Foundation.
“Being great friends, ever since we met in the Kalahari desert three years ago where we ran our first ever multi stage race, we wanted to share this adventure together.
“I am really looking forward to sharing this adventure with her and raising lots of money for Pink Ribbon!”.
Marina added: “This will be my biggest challenge yet where I will once again be pushing my body to its limits to see what it is truly capable of. While most people might call me mad, I feel privileged to be able to do something amazing with such a great friend. I was instantly up for it when Lucja hatched the idea.
“The response from the public, money raised and support along the way will be extremely rewarding. I can’t wait!”
For further information on donating to the ladies #500kin5days challenge visit the JustGiving page https://www.justgiving.com/Simply-Runderful
Runderwear, makers of running underwear, are also donating 10 per cent of the sale of any of the pink hipster brief and crop top to The Pink Ribbon Foundation as well to really get behind such a great cause.