[@Surtr] I'll give it a go, I avoided it more out of the worry that I might get the science wrong

I'd say when he's a reasonable distance away from the mountain he could clock at around 90kmh or so, which unless i'm wrong is just fast enough to run across the surface of water. This dwindling down the further he is from the mountain naturally. When he's close to it (and has been building up his speed) I see it more as a kind of overclocking thing so while he *can* go fast enough to slow time down around him (which is christ knows how many machs) he could only go as fast as his body is able to take which it wouldnt be able to stand much of in that instance.