Hi Dan
I would firstly say call the childrens charity and see whether your application has gone through and been accepted? If this has gone through, then they will support you with a fundraising pack and I would imagine, set you a minimum amount of sponsorship they would like you to try and raise. Dont panic about this though, as a charity they can help advise you how to achieve this and it is not has hard as it sounds, honest! Whether they fund the amin cost of the race is a specific charity question, every charity is different and you need to ask them specifically about that.
If you find out your application has not gone thorugh and you want to run for Cancer Research, then look on their website at their event pages and they will provide you with links and numbers to call to sign up.
Hope this helps!
Kelly