I have been challenged to do a 20 mile walk for a charity that is very dear to me. I am very unfit and it would be a huge challenge to me. I never ask for sponsorship, but I've read so many threads on here where posters complain about being asked for sponsorship that I'm quite nervous about it. I really wouldn't be holding people to ransom over it, but I want it to be worth the effort. If I'm only going to raise £20 then frankly I'd rather donate the £20! 
Be nice!