I was going to simply agree with what all the previous posters said. Namely, that speed depends too much on terrain to really be able to say. Then I saw that you are from Chicago! Being active on the Chicago cycling scene, and thereby having ridden with lots of different clubs, shops, etc. I think I can answer your question.
Most (sane) club rides around here average 18-22mph. Keep in mind that this is in a relatively large group where the benefit of the draft is substantial. There are several race training rides that are faster, averaging in the order of 22-27mph, but these are usually dominated by upper category male racers.
The local racing crowd (women) can usually maintain 20mph cruising speeds on calm non-windy days for a reasonable amount of time (1-1.5hrs) without the benefit of the draft. However, wind in Chicago is definitely a factor, and riding into a head wind is like riding uphill and will slow you waaaay down. On windy days I don't really worry about speed, but just try to keep my effort steady (constant power output if you have the luxury of a power meter).
To build strength/power, I do simulated low cadence hill climbs indoors on the trainer during the off-season, and regular interval workouts outdoors throughout the season. And I ride A LOT!!!!!!
Are you riding with any groups/clubs/shops? If you have any questions relating to the Chicago cycling scene, you can respond to this thread or send me a PM.