Can you just take a couple of hours one of these days and ride the route on your own as a test? That way you have an assessment of where you are at this time, and that - along with learning more about this group's goals and practices - will tell you whether your current fitness and goals are in line with theirs.

You mentioned that you ride 15-16mph on your own - given the reduced effort when you ride in a paceline, I would guess that puts you in a comfortable average of 18-19mph when riding in a group. Of course it's a wild guess, not exact numbers - but especially if it's a good group (i.e. people who know what they are doing), they will make extra work to keep you protected while you learn.
Usually when there is a new cyclist in the group/team, the group takes extra share of workload and allows the new people to progressively participate to the pulls - so that they can learn without being overloaded on the first day. If the group is big enough they may even assign you a companion (or 2) for the first few rides - someone who rides in the group but has the specific task to protect you during the first rides. For example if you need to slow down and recover, or to take a break, he/she would then pull you back to the middle of the paceline and so on. And, those would also be the people who give you some hints and advice to help you improve your skills.

But really the best bet is trying out the route to see where you stand, and then try a group ride with them. If it works, you have found your new group - if not, chances are that some of those people may have been around bikes long enough to be able to recommend another group that is more in line with your goals.
Good luck!

Hey Marni, we are almost neighbors!!!
There's actually a bunch of good group rides in your area... I'm too lazy to drive up in the mornings, especially when I'm post-call... but some of my buddies ride around Katy and they have a lot of fun.
Are you doing the flatland century?