There was an interesting discussion on this in the "ScienceOfSport" podcast.
According to Prof Tucker, the fastest male at any given distance will always be faster than the fastest woman at that same distance.
The interesting part is that he also says, "it depends who your comparing against". If you pick a male and female who are matched at the half-marathon distance. The female will probably be the faster of the two over marathon, while the male will probably be the faster of the two over 10k.

It's at about 1h20min in this episode:
Science of sport podcast, series.5 ep.19