This is one of the top questions that novice racers ask, but there really is no simple answer. The thing is, in road racing (not triathlon, not time trialing) it's about way more than simply average speed. It's about tactics, recovery, ability to respond to accelerations, bike handling - lots and lots of things. Unless you are that person who can easily just ride away from everyone else, there really is no predicting how well you'll do in a race by trying to compare it with a training ride, so don't worry about it, just get out there and experience it.