My own experience raising money for charity through a bike ride was pretty disheartening, I started out feeling like I was helping needy children and ended feeling like a freeloader with a bill for my monthly salary that I couldn't afford to pay.
That was fifteen years ago, and with many friends who've worked in the charity industry I understand the need for it, but I can't say I like it. And every person phoning me to sign up a direct debit so they can "get their cut" makes me sick.
I would suggest entering a wiggle sportive event yourself, something £30-£60, and raising money for charity anyway. That way there is no pressure, no target , just charity.
Or, if you're feeling flush, make one big donation yourself to cover a charity entry entirely(!)
Good luck, and good luck with raising funds! Once you've made your decision you can forget all the negativity!