I like this idea. I can see some good metrics out of this.
Perhaps to flush out the idea a bit further, what you describe I would call effort. And should include time on course, speed (or pace) and HR as well.
What I would call difficulty would be length of the course as well as elevation changes. This would be just on the course itself and not on the effort it would take an individual to complete it. Sort of an independent evaluation of the difficulty of a course overall.
Effort (as described above) would take into consideration the difficulty.
