Definitely set up a JustGiving page (or equivalent). Getting people to sign a sponsorship form is easy, but then getting the money from them can be a royal pain. So if you have the online donation option people can sponsor you instantly. Still not the easiest thing in the world, but at least you don't have to chase them twice.
When I ran the London Marathon in 2009, I managed to raise about £1,700 (my target was about £2K). A lot of that came from the good people of blonde who were very generous, and more generous than most other communities I'm part of (or care less about the odd £20 or £50 than most).
£10K is a big target to aim for, and without business donations you'll need a lot of publicity and a lot of hard work to get to that amount. Depends on the sort of networks you're part of, and the number of people you know.
Good luck with it though fella, and make sure you hassle me for some money (I'll do the same when I run London again next year

)