Flat, no wind: 20mph
Flat, moderate wind: 18mph
Moderate hills (50 feet elev. per mile): 17.5mph
Hilly (100 feet elev. per mile): 16mph

All above based on a 40 mile ride. This is the difference hills make, for me at least.

With competition in mind though: it's good to have fitness, but more important to race smart and be able to draft well, do as little work as possible when not necessary, follow the right wheels etc.