The New Pyjamas campaign has secured 15 spaces, meaning those who sign up in time can register for free, providing they raise money for the appeal.
The annual event at the Meadows on 7 March is expected to attract more than 1,000 runners.
One person running on behalf of the campaign is New Pyjamas intern Nairn Marnie, 22, from Morningside.
"The furthest I've ever run is up and down the football pitch for 90 minutes," said the student, who will run the event in his pyjamas. "I started my training yesterday and ran until I was tired.
"I might have to do some running in my pyjamas to make sure that they won't give me any problems on the big day."
To get involved, contact Patricia Hess on 07500-829 226 or visit www.newpyjamas.org.