Congratulations on taking this on! I am walking a half marathon for Leukemia & Lymphoma Society on June 23 - and now it seems short compared to the 3 day event.
I second all the advice you've been given, esp as to wearing wicking socks. Also drink lots of water. Don't know what they do about bathrooms on the 3 day walks. I know ours has porta-potties along the route. I've been doing long walks for training, and I have to plan my route around bathrooms! I hate trying to find a place to duck off the road
I did find the fund-raising aspect rather daunting to start, but we had several meetings about how to do it, plus we've done several team events. We had a spaghetti feed with a silent auction, and have a garage/yard sale on 5/19, at which we'll be doing a treadmill marathon (with several walkers/runners) that people can donate to. I actually have raised my goal, plus more than 50% more, most of it from family and friends. My advice is to write a great letter and send it to everyone you know. I was amazed by the response. Good luck.