First don't worry so much about the speed. Are you getting a good workout ? Blowing off Stress ? Getting the Heart Rate and Breathing up ? If so that's by far the most important part. Back to average speeds, it's hard to compare average speeds because of the variables, everyones routes are different(hills vs flats), their height above sea level, their cardio pulmonary capacity is different, and the type of bike (eg beach cruiser vs roadbike with skinny tires). That said I had someone say something to me earlier this year that kind of sank home. "If you want to ride faster then you have to ride faster" sounded kind of lame when I first heard it but when he explained to me that I needed spin faster and use larger gears it made sense. I've made a conscious effort to push large gears longer and spin faster this season and I've seen a pretty dramatic increase in my average speed. Something else I have done is to take the mountain bike with knobbies out on the road as it provides more rolling resistance and a better workout. Can I also suggest finding someone slightly faster to ride with ? Go find a social ride with a group somewhere as they usually stop to re-group often and there is no competition so you don't get left out alone.
Shog