You are doing fine. Unless you want to start training for racing or have a need for speed, your average will go up with experience, time, and maybe a little specific effort. When i started cycling, my average was probably around 12. That was on a mountain bike with slicks. After I got my first road bike, I did work on improving, and it went up to about 13.5. Now it's about 15.5, but the reason it isn't going up higher has to do with the kind of riding I do and who I ride with. I live in an area that's not flat, mostly rolling, but also with some hard climbs. When I ride by myself or my husband on shorter rides (15-25 miles) my average tends to be between 15.5 and 16. What's surprised me is that it's creeped up, even after moving to a house where every ride ends with a 10-15% grade climb. But, when I ride longer distances, I go slower. Perhaps this is self defeating, but I am riding for fitness and fun, not just speed. I also ride with people who tend to average around 13-14. That's fine with me, especially, if it's after a day that i have done a harder ride. So my average goes down. If you look at my Bike Journal stats, it will probably say my average is 14.7 for the year, a result of this type of riding.
The club rides I lead are advertised as intermediate; the average tends to be around 14. Sometimes there are slower people and they are fine. We just wait at the tops of the climbs for them. This is social riding, not hammering. I finally found a group ride where I can keep up and even be in the front. The average is around 15. I usually drop almost everyone except the leader on the hills, but I am a slow descender, so I lose there.
So, unless you have a need for more speed right now, give it time and a little work and you will improve.