Just add to your mental preparation that there may be riders across the whole range -- experienced, group, organized riders and those who are very new and all in-between. Try to keep your distance from others, look around often so you know as much as possible who else is around you and watch the riders you are coming up on - are they weaving (squirrely), are they riding smooth and steady. If you are moving one way or the other use hand signals, if safe to do so. I use hand signals when riding on the road and I need to move from the shoulder into the traffic lane, even if I know no car is around, because you just don't know.

Have an awesome ride and enjoy it.