I did Marlow Duathlon yesterday and the weather was ok for running (well, sort of...) but it was bloody miserable on the bike! Unfortunately it's not always going to be 12ºC with light cloud cover and a gentle westerly breeze.
The worst I've encountered was at Brockenhurst last March, when the temperature dropped from 9º to 2º between leaving home and starting the race. In a vest. And then the rain came down. And then the wind got up. St Johns were very busy at the end of that one.
It's always worse for runners with less racing experience, because until you've done a few in a variety of conditions you just don't have a clue how hot or cold you're going to get.