I ran a really downhill 8k yesterday. It was so downhill that, even at at 8000', I ran the race about 20 seconds per mile faster than 5k PR pace, despite the fact that I'm not within 30 seconds of 5k PR shape right now. After the race, my legs were about as beat up as they are after a typical marathon. Why is this? Is it just that I'm not used to running so fast (I never race anything shorter than 5k, so I never really run super fast) or is it something to do with the downhill pounding? My entire body is pretty trashed still today and I expect it to take a few days to feel normal again. I've done some stupid things in shorter races in the past (taken a 10k out and 1s off 5k PR pace and blown up horribly, etc), but I've never before felt so trashed by anything other than a marathon.