I did the 3-Day in November of this past year. WOW! What an experience!! It was the most physically difficult thing I have ever done but it was also the most inspiring!! I think everyone should do it once!
You said that you run quite a bit each week. To be honest with you, that really doesn't matter too much when it comes to the 3-Day. It will aid in you endurance but that is about it. There are a million resources for you on the website. They recommend a 26-week (ithink 26 weeks) training schedule. It seems like a long time to train. Don't overestimate your ability!! I didn't take my training as seriously as I should have and boy was I regretting that by the end of day 1!! Commit to training and you will do great.
The fundraising is pretty easy; much easier than I thought. Consider joining a team in your area. The comradery is great and they also (if you have a 'team-minded' team) help everyone to their fundraising goals. They will alos do group walks which help you get a sense of how far 20 miles really is.
If you have any other questions, please PM me.
You will experience every emotion possible during those 60 miles. I know I experienced them all over again during the final 2!! It is amazing, moving, uplifting and inspiring! Do it!!
I wish I could bottle this feeling and sell it!!