I wrote an answer to this last night and then clicked the wrong button and lost it all. Oh well.
The first thing I think of with organized rides is flat tires. I'm always amazed by the number of people who have flat tires within the first mile of an organized ride. This happens even when there's no debris anywhere on the road, so I tend to think many people have not been riding regularly and/or have been ignoring their bike maintenance. So the first thing I'd say is to make sure everything about your bike is in good working order before the ride.
Second, make sure to plan for the whole day, not just the actual riding part. You'll be parking in a big lot or field, and you might have a bit of a walk from your car to the sign-in area, restrooms etc. It might be crowded, there might be traffic on the surrounding roads, there might be a line for the restroom. So you'll want to make sure you leave yourself enough time at the ride start.
I always like to make a checklist in advance of the ride so I don't forget to bring anything with me. Start with 1. shoes and 2. helmet. Include everything you'll want to wear (head to toe to fingertips) and anything that you'll be carrying in your pockets or bike bag. I also bring extra clothes that I might decide to switch to at the ride start, like an extra layer if it might be cold or a lighter weight jersey if it might be warmer than I expect. Also clean clothes to change into after the ride.
Other than that, have fun and enjoy the ride!!
p.s. related to Oak's post, they usually (but not always) serve Gatorade at rest stops.
- Gray 2010 carbon WSD road bike, Rivet Independence saddle
- Red hardtail 26" aluminum mountain bike, Bontrager Evoke WSD saddle
- Royal blue 2018 aluminum gravel bike, Rivet Pearl saddle
Gone but not forgotten:
- Silver 2003 aluminum road bike
- Two awesome worn out Juliana saddles