That's a good observation, and I think it holds for almost all anecdotal evidence I can muster. It might be because of the types of girls/guys that are attracted to distance running, specifically competitive distance running or from inherent differences in modes of thought between the sexes.
My mental model of a 'hypercompetitive' male is an American Psycho/wolf of wall street figure, ruthlessly crushing others so that he can be the best. On the other hand, when I think of a competitive woman I think of a perfectionist - honing her profession until she is the best. It's easy to see how both sides of this could be manifested in competitive distance runners. Obviously there are exceptions to this I guess the next question would be why do competitive males and females differ drastically in behavior? I would try to provide an answer but I'm sure that it would sound like some sort of 'armchair evolutionary psychology', and my mental model is probably tainted by the media (as evidenced by my reference to popular films).
I believe that there is a passage in Gravity's Rainbow that compares the thinking of males and females to the analytic/synthetic distinction - Males tend to analyze, separating things into atoms, trying to find general principles from these atoms whereas women proceed from thesis to antithesis to synthesis via dialectic. One can readily proceed from the analyst to the neurotic, hypercompetitive male and from a neverending synthesis to a perfectionist. There's no dialectic in trying to be the best runner in the race, but there's something like it in trying to improve your performance from race to race. Similarly, there aren't many different atoms to identify across ones history of performances but there are a hell of a lot when comparing the self to others in a race. Again, as to why this difference of thinking exists I have no idea, it might not even be a question we can answer.
On another note - The 'free spirited' distance runner is another stereotype but thinking back to all of the clubs/teams I've ran with it is much more of a male phenomenon. Why would this be?