I'm not so sure that it's mostly socialization. We've had relative gender equality in sports for several decades, and the male/female performance gaps haven't moved much since the 80s. Some girls and women just aren't interested in playing sports; many gravitate towards other physcial activities like dance and gymnastics (still a sport, but one that is much more popular among girls).
The other thing is that, in a speed/efficiency-based sport like distance running, talent is related to morphology. Not many women are shaped liked elite distance runners. Obviously, the sex performance gap is less with distance running than with many other sports, but this does not mean that the distribution of talent within the male and female category is the same. It's possible that fewer women have elite/sub-elite morphology than men. I don't know if this is the case, but it makes sense on a hypothetical level. While many women with relatively wide hips and large breasts can hit/kick a ball, throw objects, etc. just as well as or better than women with narrow hips and small breasts, they're not going to be as biomechanically efficient, so they're less likely to be sub-elite or elite than somebody with these characteristics. A simple way of saying this is that a straight up and down body is more efficient than a curvy one, and there's good reason to believe that there are more straight up and down men than women. This might lead to a situation where there's a 10-11% difference between the performance of top males and females but a 13-15% difference in performance between those who place, say, 500th or even 1000th in their sex category in a marathon.