Pre-ride prep ...
So much to discuss here, but let's begin with the bottom-line ... how far do you plan to cycle next?
Distance and amount of previous cycling really determine how much prep you need to pursue.
Hydrate ... the longer you plan to be on the bike, the more well hydrated you need to be when you START your next ride.
Planned power output ... you know whether you can comfortably ride at 16mph so if you plan to ride with the 18mph group, you've got some energy requirements to plan for. That could involve some meal planning, consuming such about an hour before your ride.
Solo event ... you have to do all the pulling yourself. This is a mental preparation item. Know the weather, know the wind, select a route where you can achieve your objectives (e.g. speed, destination).
Team event ... mentally prepare for riding with others whether in small groups or large, long pace-lines. You should be able to ride further and faster with a group than solo. Don't be afraid to select a longer route than you've done before if you know a few others will be helping pull you around.
Previous cycling ... attempting a century ride while only having stayed on a bike ~2hrs previously during the season is not advised. You may be able to do it ... once. And then have some consequences the next day or so. You don't have to have ridden a 100k ride in order to attempt a 100k ride, but you should have completed a couple of 50 milers so you know how your body responds in the given conditions. Word to wise: work your way up in distance and speed. Much of this prep, again, is mental, not just physical.
Mentality ... I've already mentioned the mental aspect of cycling a few times. This cannot be overstressed. Have a picture in your mind of your ride, set goals (even simple ones like "I plan to make it back home in one piece"), and think about what you'll do post-ride. Cycling is a physical exercise but the requirements mentally are just as demanding. But don't forget ... the experience of being outside, sometimes with others, is a mental joy as well.