From the Massapequa Park Bicycle Club (Long Island, NY) website

Cruising speed in miles
per hour on flat road.
This is NOT the ride average.
Open*: race training
AA rides: 20-23 mph
A rides: 17-20 mph
B rides: 14-17 mph
C rides: 11-14 mph

And just for giggles, the US Cycling licensed category definitions as it equates to training time:

Cat 1: Job
Cat 2: Job, Car
Cat 3: Job, Car, Spouse
Cat 4: Job, Car, Spouse, House
Cat 5: Job, Car, Spouse, House, Kids