Riding with Faster Groups
If I'm telling me more than you want to know, just tell me go away;)
I've been thinking about your query a lot over the past few days because I am currently working with a woman in a similar situation. She's 50+, been riding for a few years, and was comfortable at 15-17mph, but wanted to start attending some of the "training rides" (generally 18-22 mph).
The big thing we worked on (and the thing that she now says helped her the most) was her cadence. We upped her to about 90-100 rpms and now she is able to stay on top of her gear and respond to changes in speed by the pack. It's also helped her climbing considerably.
The other thing that seems to help, is that we found a "good space" for her within the group. If she is always hanging-on at the back, she tends to get dropped. We put her in the sweet spot in the middle of the pack and that seems to work better. For now she just pulls-through when she gets rotated to the front.
Generally, if you are the slowest rider in the group, they would much rather have you pull-through than take a heroic turn at the front and then slow the group as you struggle to catch back on.
If you are riding regularly with the same people, let them know that you are trying to improve. Most riders will make accomodations for a new(er) rider looking to make gains in fitness and skill.