Mmmm, have fun! That's most important
Are you going to be riding iwth any one, or going solo?
You'll be starting with a big group of folks, and it will take a while for the pack to thin out as everyone tries to find their little spot. Be patient while this happens, it can take a few miles for everyone to settle in. And since you start in a big group, you have to remember not to make sudden movements or stops without looking around and announcing. Be aware of other riders movements ttoo. once you get going, it's kind of fun to chat with other groups of riders as you pass each other. folks are usually pretty friendly on these rides. Be sure to call out any obstacles you see on the road for riders behind you, and announce yourself when you go to pass other riders. Don't get too close to the rider in front of you, since you don't know their riding habits. There will probably be some intersections with police who will wave large groups of riders through, which is always nice.
What is the next ride up? 40 or 50 miles? Any thoughts of trying that? It's a good time to try a longer distance since there will be support (rest stops will have food and water for you, SAG vehicles to help if you have mecanical problems).
Probably need to get there early too. Not sure where you are, but parking can sometimes be tricky at these events.
Have fun! You'll have to report back.