Long Runs

Today I had a run of just under 11 miles, and it really took it out of me.  I actually feel pretty good right now, but my pace was really slow - much slower than I have run other long runs.  I'm trying to figure out if that's because of the length of time since my last run (almost two weeks), the relative high altitude (nearly 3600 feet at it's highest points) or the difficulty of the trail.  I tend to think that it's some combination of all three of these - especially the trail.  The Boone Fork's Trail is a five-mile loop that's much more difficult than other trails that I've run...there are more exposed roots and rocks than can be counted, more cliff climbs than I care to remember, and the general muddy conditions.  This is a much more difficult trail than the Osmanthus Trail that I run in Virginia Beach.  On Osmanthus, I can average between 9 and 9 1/2 minutes per mile.  On Boone Fork, I average between 11 and 11 1/2 minutes per mile.

I must say, I think I need to start working on my speed, but there's got to be something that makes Boone Fork so much more difficult.  I wonder if there's some sort of trail rating system that gauges the relative difficulty of trails on some sort of scale?