I just ran yesterday at Tokyo. The cold wind and rain (plus feeling tight hamstrings after a recent pulled hamstring injury) slowed me down to a 2:52 off a PR of 2:42 at NYC last year. I too optimistic like many Japanese runners as I went out at 1:21:xx for the first half. I felt hypothermic too but did not require medical tent. I ran a 2:54 for Boston last year (no injury just wrecked by the cold weather and ended up at the medical tent with hypothermia). Some runners do better in this weather.. a friend of mine ran a PR of 2:44 yesterday after running 2:48 both at NYC and Boston last year. He says he does relatively better in colder weather.