The only one I did was the 5K "Dirty Girl" Mud Run charity race. It was held at a ski area so there was an awful lot of uphill running (seemed like it was more uphill than downhill). Since it was such a short race there was no training needed, but I probably could have used some for a few of the obstacles (the rope climb and the net climb). My biggest problem was I wore the wrong footwear- I wore Keen sandals instead of shoes and got blisters from the mud rubbing between my feet and sandals. I'm planning to do it again next month.