Hi Pedalt.u.p...
There are some factors -- age group is also one of them. Racing in the midwest at least (WI) - avg speed for women probably between 30-50 will be about 20mph. To win -- more like 25mph. Terrain -- ie, a hilly race like will yield a lower average. Your best bet is to check out websites of races you are interested in and see if they post the results. Usually they post avg speeds. This lets you see if you can be competitive (when you want to), or whether you just want to ride it to finish.

Go out there and have fun!