If you are familiar with running, biking is not so much different. It really isn't a matter of how fast you go, but if you do the same route multiple times, comparing your times from attempt to attempt.
After I moved to a new area last year, I found a 25 mile loop that I could do in about 1:30 or so. I was fairly disappointed because I was able to do a 25 mile loop in my previous town in about 1:15. However, my previous town was basically dead flat, and my current loop has about 1200 feet of climbing in it. Too much variability to just say I should be able to go 20 mph.
Depending on terrain and conditions, most of my rides average between 16 and 20 mph when I ride alone. The speeds go up fairly quickly when with others. To give you a comparison, I am 40 years old, in fairly good shape and am able to tough it out at the back end of the "fast group" in my club rides, but I wouldn't take any trophies home in races.