Hi Jenni -
Hey, don't worry, you're plenty fast. Slowpokes like me would have a tough time keeping up with you! (So, try not to drop everyone else
)
Also, unless this is an "event" sort of ride (a ride with a large number of people participating, where people pay to participate, generally in advance) then there likely won't be markings on the road. I might have misunderstood, though.
But you might get what's called a "cue sheet" (or a "route sheet"), which lists the streets to turn on and the mileage between each turn. If you have a way to attach your cue sheet onto your handlebar (just a small clip attached to velcro works fine), that's a lot easier than stuffing it in your back pocket, but of course its not essential. You will likely hardly ever need to refer to your cue sheet if you're riding with people who know where they're going, but it's very good to have. 
Odds are good you'll find people who ride at at your pace (or, almost at your pace
) to ride with. Could you let the person who is organizing the ride at Bike Gallery know that this is your first one? The organizer could give you an idea of the general pace of the group, and perhaps even introduce you to some other riders.
Don't worry, it's not as complicated as it sounds. Get out there and have fun!
2009 Lynskey R230 Houseblend - Brooks Team Pro
2007 Rivendell Bleriot - Rivet Pearl