A lot of the charities do organised events like Cancer Research UK have a large series of 10K runs/walks around the country throughout the Summer.
If you want to raise money, a good place to look is Just Giving -
http://www.justgiving.com/ - as it's mostly online and they can sort out the tax breaks too (28% on top of what you raise, for free!)
It also means if you post elsewhere you can include it in sig files etc. which is a very good way to spread it. If you do that, post a link back here and you might well get a few sponsors.