I ride with my gf most weekends. She isn't as strong a rider as I am, but our rides are still very enjoyable for me (and her too). I will do all or almost all of the pulling so she can improve her leg speed and get used to riding a higher pace, and we can also ride longer distances. When she first started, I slowly educated her on how to ride in a group/paceline, suggested gearing/cadence, suggested equipment needs, etc.
I think if your gf enjoys riding, you might suggest doing an organized ride together (or at least mention how much you enjoy them). The ride can be one of the shorter routes on a century ride -- just something for her to look forward to/get excited about, and learn more about what it takes to improve. Once my gf decided she'd like to try a century ride she really got into the whole cycling thing. Our first organized ride together was a 30-miler, then a 70-miler. I know she really enjoys the "challenge" of an organized ride and definitely works on preparing properly for each one. I've also found that she will ask me more questions and want to learn more when she's "in training." Now, we regularly ride centuries together and she truly enjoys the rides and the work that goes into preparing for them!
As for you, I would suggest you do some things to make your "together" rides more challenging/beneficial, like do some "big gear" work, or work on increasing your cadence, or work on pedaling out of the saddle -- there are lots of things you can do. You can also extend your ride before or after your "together" rides -- get creative -- it's worth it!
Good luck and keep us posted!