I used to do a lot of running on the AT. Almost all of PA has a surface of cobbles making it very unstable for running. Lots of foot traffic has worn away the dirt between the rocks.
I was pretty surprised by the runnability of the first section, knowing the reputation of the Trail in PA. I don’t think the rocks in this section have had dirt between them for many thousands of years, if ever. And why would you chose to route the trail across the Knife’s Edge rather than 50m parallel to it (answer: because whenever there’s a choice, th AT always goes up).