We've done the C2C and London to Paris and we've done the planning ourselves (the planning is part of the fun). I agree with Raymond, you raise what you can, not a set amount. We've raised around £1000 over the 2 rides, not a fortune but all the charities we've helped have been happy.
We've always used Just Giving, and I know they get a fee, but people feel safe giving to them and once your page is set up you'll feel committed to the ride.