I have a 40 pound downhill bike that I ride on all terrains, mostly road, still with DH tires. Maybe it comes from the recently suppressed street BMXer in me, but I love having a heavy thing to ride around that really makes me push it and sweat it all out (Upon re-reading my post, I see this comment to be very suggestive. Get your mind out of the gutter!- and I'm proud to say that I have not yet once walked the bike up a hill yet (on road).
I don't have a gps device, but from simple time/distance calculations I seem to average between 10-15mph depending on the route/day. Distance-wise, I usually aim for 20-25 miles on weekdays and 35-45 on sats & suns. Going to actually attempt PA and back after work on friday! If you don't hear from me on Saturday... well, just assume I'm dead.
All in all, I love being able to choose between dirt, rock, or road at will. Though I spend more time on the road simply because I ride it to and from the trails, the convenience of having a mountain-monster at my finger tips outweighs the inconvenience of full suspension and knobby tires sucking up all of my torque!