Ask the ride leaders in advance about the ride. By this time of year, a ride labeled "16 mph" might be a lot faster, since all the regular riders have been riding all summer.
Start with a ride that sounds "too easy" and see how you fit in.
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About
"C" groups / social rides / casual pace rides:
Here's a "
16-17 mph on the flat roads" group ride. And the average speed for the whole ride is probably around
14 mph, due to hill climbing and slowing for stop lights.
These slower groups tend to leave more room between riders, since the draft isn't as critical. But riders still get bunched up at times. So holding your line, and riding as if someone is always 6 inches from your back wheel is always a good idea.
(Some of these riders have been riding for years, and can maintain the 16-17 pace all day. New riders sometimes don't have enough practice with longer rides to keep going more than a half hour or so.)
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I do two types of group rides.
Easier pace, longer distance, scenic rides with a good group of riders. There's so many good routes that the local cyclists have refined over the years, and passed down from one rider to the next.
and
Rides that are just a little too fast for me, hanging on in the draft. It's a blast. I can't motivate myself to work this hard when I'm riding solo. I'm sometimes one of the last up the climbs, but not by more than 10 or 15 seconds. I don't like being "that guy" that everyone keeps waiting for.