He will be joined by Colin Chapman, Paul May and Colin Thomson in the Everest adventure. All four are into outdoor adventure. Their aim is to finish the marathon, rather than come first.
"We have agreed to start and finish together and help each other through it."
In the Himalayas altitude will be an extra challenge. Simpson had headaches and nausea when he tried the equivalent of moving from sea level to 5000m in altitude chambers.
The men leave for the world's highest mountain on May 14. They will have time to acclimatise and do some trekking before the marathon begins.
People can sponsor them for any amount of money. Corporates and businesses can buy a prayer flag for $3500, have their logo printed on one side and the wish from a heart kid on the other.
The flags will be flown at base camp, then cut up, framed and given to sponsors after the marathon. The children will get a photograph of their wish flying.
So far the group - known as Good Family Chaps - has raised more than $140,000. Their trip is self-funded, so all sponsorship money will go to the charities.
Heart Kids was chosen as a charity because heart defects are New Zealand's most common congenital problem. An average of 12 babies are born with them every week.
The New Zealand Spinal Trust helps in rehabilitation, information, research and support for people with spinal cord injuries.
People can donate to whichever charity they choose, and sponsor whichever of the group they prefer.
To donate or find out more, go to Running Around Everest.