And how even and exact they are. A runner's form.
As a former distance runner, I agree. On video this looks like the stride of a distance runner (or middle distance, milers, etc). long strides with an easy back kick that aren't particularly piston-like (which would be more of a sprinter's gait- this form looks more like the way someone who trained as a distance runner in high school or college would run imo). Hard to tell how the video might distort the way the stride looks, but to me this stride is similar to that of many young male distance runners doing a track workout. I definitely believe this is a man now.