15 to 19+ is a HUGE jump. Find a group ride with a more appropriate pace. One of the local clubs has multiple groups/speeds for their weeknight group rides. I may still be riding in a hammerfest, but I'm riding in the SLOW hammerfest. (If I wanted to ride solo, I'd grab a cue sheet and take off on my own.)
There are several clubs/groups/shop rides in my area that organize rides with distance and speed appropriate for just about any type of rider. It's worth taking the time to find them. Find the ones in your area (it may take some asking around) and see which ones work for you. You'll find all ages and experience levels present. I've noticed that the social rides (where the post-ride lunch spot is a big part of the selection) have a more mellow pace than the "training" rides. If the ride info lists an email or phone for the ride leader, call/email them ahead of time so you have an idea what to expect.