I tried the hand position early in this thread this morning. I must say my back did not like it. Despite the fact that an mtb presents your body like a sail, thus slowing (or helping!) the bikes speed it is most comfortable. When I was a teenager I rode my Raleigh (forgotten the model) road bike into the ground. At 39? My body doesn't enjoy being aero nearly so much.

35km/h for 3-4 hours (on a good road with no obstacles to slow me down) is quite possible. Try and go 36-38km/h and I can do it, but for 30 minutes at best. Try to go faster? Err a few minutes and scottie is yelling "Ye canne break the laws of physics!"