Slower paced rides almost never rotate through as a pace line. They tend to stay farther apart, too. I'm assuming that your ride is one of these, since it's "just" 25 miles. Let the ride leader know you are new to group riding, but don't worry about it. It'll be fun.

The main thing to know: assume there's someone right behind you, so don't swerve or brake suddenly. You'll find out about hand signals or verbal signals by example during the ride.

Typical rides in my area. These are flat road speeds. Average speeds for the ride will be lower, due to hills and slowing for stop lights.

12-13 mph. A "smell the roses" type of ride. very casual pace.
14-15 mph. An easy pace, no drafting. Riders often are able to hold these speeds for a couple of hours, surprising very new riders that aren't quite ready.
15-17 mph starting to pick up the pace. Mostly experienced riders, but new ones fit in easily. Few stops and longer distances. Still not much drafting.
18-19 mph OK, now the drafting is starting to be critical for riders that aren't quite as strong as the ones in front. Often no formal rotation--strong riders at the front, others in the draft.
20-22 mph Rotating through, pushing hard on the hills too.
22+ (and more!) mph the fast ride.