When I ride my heavy, heavy mountain bike type bike, I average around 12 mph. If I'm riding one of my lighter bikes with skinnier 700c wheels, I go a lot faster. But honestly, I don't care. I had computers on my bikes and I took them off. I ride because it feels good and I want the exercise. How fast I go only matters to me inasmuch as I need to plan ahead enough to be somewhere by a certain time.
My advice is this: don't worry about how fast you go. Just enjoy the ride! If worry too much about going faster or longer or whatever, you might just ruin it. But that's just me. I know a lot of people enjoy keeping track of their stats.