I always forget how running downhill is a bit harder than I always expect it to be -- a lot of stress on the quads to catch yourself over and over again.

It does feel good to just burn past people going downhill though, if you're used to it. I ran the Whistler (mountainous skiing town over this way) Half Marathon a few years back in around 1:45:00 or something like that, and made up most of my time on the downhill segments. Lots of fun, but it punished my legs doing it.