I don't ride that much. Probably 100 miles a week. Sometimes more, but often less.
On flat routes on a flat bar bike, my cruising speed with moderate/average effort is between 16 to 18 mph.
On a road bike, typical pace is around 2 mph faster. However, I've never averaged more than 17 mph on any ride on this route even though the terrain is quite flat. This is probably because there are at least 15 stops and/or sections where I must slow at trail intersections in the first 5 miles.
There are few stops or slow down sections over the next 10 miles but it's very windy, often with 15 to 20 mph winds.
On group rides, my effort level drops drastically. At 14 to 16 mph, I might as well be at home sitting on the couch. 20 mph feels more like 12 mph on a flat bar bike if I'm in the pack.
Basically, the more you ride, the more efficient you will become and the faster you'll ride. Also, a set of michelins improved my efficiency noticeably as well.