Ditto to what Eden said. I have been in races where we roll around in circles at 15 mph (and I am angry and chomping at the bit, but tactically can't go faster) and other races where we average over 21 mph (these are actually from the same course, but different years!). There are too many factors to consider to give you an average speed for a race - what't the terrain, who is racing, what's the weather, etc.

Just go out and give it a try, and most importantly, have fun!

SheFly