Flat Footed - there were runners before Garmins and other modern techy gadgets you know! If he's a long-time runner he would know how long it takes him to run a given distance. Also, I bet there were lots of tools down there he could use to measure with. Many years ago I was into orienteering - one of the first things I learned was how to measure distance when cutting across country on a compass bearing. After a while you just "know" how far you've gone. I've been running for 50 years now - I always know what pace I'm running and thus how far I've gone, it's ingrained knowledge.
