> Honestly, I think the biggest factor in these longer distances is the smaller sample size. There just aren't as many elite athletes competing at the ultra distances as there are at marathon and shorter distances.

Off the top of my head, though, is that sufficient? There are many sports beside ultra-distance endurance races which are fairly niche, but the impression I get is that their elite events still don't tend to throw up female outright winners at all. I assume that ironman is fairly popular and taken quite seriously compared to most sports (as opposed to the few which get the lion's share of the attention and participation) and yet it threw up something as striking as Chrissie Wellington https://en.wikipedia.org/wiki/Chrissie_Wellington 's success.