I agree 100% with what everyone else has said. There is no "right" speed. I usually look at the average speed I am at in the early part of the season, and then set myself a goal of 1 or 2 mph higher by the end of the season (my biking season usually runs from May through September), depending on where I start out. Right now I am riding around 2 mph faster than I was in May. Then again, I decided to only bike this summer, while some years I am swimming and running for triathlon training, or almost exclusively running for marathon training. That makes it harder to really work on my biking speed (and it has definitely suffered over the past few years). Actually, for the past 2 or 3 years I thought I would be forever stuck at a particular average speed, but now I've nearly clawed my way back to where I used to be. A week ago I was able to ride 100 miles by myself at about the same average speed I was riding 20 miles a few months ago, struggling to try to draft behind other people.

Practice, practice, practice! And have fun, fun, fun!!