I always think average speed is a bit of a misnomer tbh. If you are riding in traffic, and dealing with junctions, lights and other street furniture you're forever slowing, stopping and starting again. Where I live, South London, it's at least 10 miles before I hit roads that I can actually get going on. The first and last ten miles are like a warm-up/warm down! The thing is just to find a pace that your comfortable with, you'll soon be able to build up your stamina and pace with time, and as Ian says it's a lot quicker if you're riding with a mate/group, and softer on the legs too
Today I did a 60 mile ride, alone, it was pretty hilly (up and down) and averaged 17/18 miles, which is normally what I seem to end up with, which seems OK.