Young fit Gendry doing around 5 miles and hour makes 30miles in 6 hours. Lets look at it this way Oprah ran a marathon in under 5.
It was a challenge for Sansa to do her best, and by turning her back she was saying that Sansas best was not good enough, she has no fear of her.
I am curious, but I have to be careful I have been called a troll here, some one had even said their inner Areola was about to come out over something I said. I am not even sure what that means. Anyway why does every one think Arya knows what she is doing when it comes to back room politics. Is that something one would learn to be a faceless man. We know they learn to lie, disguise ones self, fight, kill and all that, but why would they learn an thing about underhanded politics?