on a level road, no wind and no hills, you could probably sprint to 40+ without too much training.

of course, i don't know where you'll ever find such a place lol. most of the time i average about 18-20 mph, but i live at altitude and ride in a mountainous area. top speed ever? 65 mph coming down a 10% grade with a tailwind. some scary **** ...

btw, welcome to road cycling. i too started out as a mountain biker, but gradually came to love the road.