I agree with onetomanycornettos.
You have your very specific reason for donating to this charity (sorry for your loss). Donating to charity is a personal choice. It is YOUR choice. You cannot at the same time expect other people to support the same charity, and on top of it, part with money to it.
I dont like the "sponsorship mentality". It says "I want to support this charity. I want to run a marathon/bunge jump/parachute jump etc, now YOU pay me to do it."
So what that your husband is in training? So what that it is a big thing, why should your friends pay?
I am not surprised things are getting a little strained. I bet they cant wait for it all to be over and they can be "safe" around you again. Sorry if that is harsh, but I think it would be absolutely AWFUL if my friends were to pester me for sponsorship money. It is the charity of YOUR choice, so YOU pay or volunteer, or whatever you do, it is nothing to do with your mates.
Personally, I volunteer in the church, I am also involved in voluntary work for the parish newsletter, has been for years. I also donate to red cross. dh gives up his free time to be part of the mountain rescue team. he also spends several hours a month teaching computer skills to senior citizens. I think that is doing a lot of good. But your husband running a marathon, which he is doing FOR HIMSELF, and co-incidentally it raises some money that friends are badgered into giving, that is a different matter alltogether. It is a smug middle class "I set myself my own personal challenge that benefits nobody but me, and if you PAY money, it will benefit others."
Sorry, as you can see, I am SICK of being asked to sponsor this that and the other because somebody is going to walk a pram in the park, and I pay, or somebody will carry a milkbottle across richmond park, and I pay, or swim 2 yards off the coast of Brighton, or something.