My only advice is to be very mindful of those around you. Charity rides brings out all sorts of people and routinely people that have no idea how to ride in a group of people. Like yourself, no offense.

This lack of experience can make things dangerous. I would not get to close to bunches of riders and watch when you're going around turns as many people don't take the proper line and will cut you off. When you pass someone, let them know in advance and give them plenty of room when you pass. Most people on these rides aren't paying a lot of attention to keeping their track straight and they tend to be all over the place.

You should have a great time and as long as you keep your attention, you should be safe at the end.