I once had an extremely strange experience whilst doing a half marathon.
I was doing it on my own, on the hottest day of the year, with only about 500mls of water held in my hand. Stupid, I know, but I was a teenager.
Therefore, the following might have been the result of dehydration...anyway, I ran about 10 miles with not too much problems apart from the heat. After I got past the 10 mile mark I developed a strange dislocation with what I was doing. It was almost as if I'd stepped back from my running self and was watching what he was doing. I saw that I was putting one foot in front of another but for some reason this struck me as rather strange, abnormal, in fact. After another ten minutes of this I saw the legs slowing down without being bothered one way or another and then suddenly I was standing there on the concrete, panting and wondering what the hell was going on
I walked the rest of the wayhome and felt fine apart from being totally bewildered at my strange experience!