I've noticed that a majority of races that I've entered explicitly state that they are not open to walking, which I imagine refers to people that opt to strictly walk instead of run. I know there are 'purists' that feel that races should only be run, but there seems to be more and more articles published (online and in magazines) that say that runners should take brisk walking breaks.
My question is at what point in a race do you feel that someone walking should not be participating at all?