The mtb will be noticeably faster with slicks, get something like Schwalbe Marathons or Continental Top Contact in 26 X 1.5" and pumpt them fairly hard.
My other recommendation would be full fenders and a brooks saddle :-)
Use the bike you have for getting around to what you really want/need, say a shorter 2-3 day tour around where you live(if possible) and keep as a commuter/grocery getter when and if you get a touring bike.
(Edit misread the original post) maybe your bike is set up for offroad riding, with the saddle a bit lower and the bars higher or nearer than would be best for road riding, my mtb is set up so i can get behind the saddle to lower my center of gravity or shift it back, and the bar is close to me and wide so i can rally wail on it when i hit sodt mud. I lose a lot of spinning power from this and wouldn't like to ride it very far with the current setup.
Check that your leg is almost fully extended at the bottomn of the pedal stroke and that youre leaning forward far enough to put some, but not too much weight on your hands.
Last edited by Black Shuck; 05-24-08 at 11:39 AM.