There were some long, relatively flat sections in the 50K I did this month. I'd always imagined that would let me go faster but I just felt heavy, slow, bored, etc. I think the variety of going up and down hills makes me feel like I'm going faster, even though I'm not, and my muscles feel fresher too - which they probably are.
I feel the same Bash. Even when I have a slower run on the trails, it feels faster that these crappy runs on the road. My feet always seem to feel lighter on the trails. :)