Okay remember a couple of things:

1. I've been doing this for 6 years now.
2. I'm training to race.
3. I'm on a tri bike, riding down in the aero position (similar to your icon, but more stretched forward over aero bars), which easily adds more speed as wind resistance is eliminated.

You're doing fine in your cycling, and that you're getting out and doing it at all is great; you're ahead of a lot of people that way. As you keep doing this, increased fitness will come. You'll discover you can ride further and faster in no time. Heaven knows I don't go anywhere near that fast on my mountain bike on city streets, nor did I go anywhere near that fast when I started, either.

What type of bike do you ride?