I agree with lots of the comments here. Your average speed shouldn't matter too much. Wind, hills, stop signs, etc. all affect average speed. As long as you're enjoying yourself - have at it!

Where I ride, it is FLAT. The only "hills" are into driveways. I run into a lot of wind here (averages 10-20 mph). I ride in town in the neighborhoods where there are a lot of stop signs, so that slows me down. My computer tells me I average 15 mph, pretty much regardless of how long I ride (10 miles or 30...). As Eden said - your average pace when you're going good is usually way different from the overall average. In between stop signs and on the long straight sections (I ride on the periphery of town for part of my loop), I average anywhere from 16-20, depending on how I'm feeling). So, my point is that there is a lot of variability within a ride. And as long as you're enjoying yourself, there's not much to be gained to compare yourself to others. As MP said - some are faster, some are slower than you. Have fun!