Any locals are folks familiar with the area able to tell me what I’d be facing with any of these?
North
https://ridewithgps.com/routes/34949648
Harpers Ferry
https://ridewithgps.com/routes/34949526
Brunswick
https://ridewithgps.com/routes/34949610
I ran the full length from PA border to VA Border this past June. Here is a link to my blog recap. I am more than willing to share my insights, but what exactly are you looking to learn? I could go on and on but want to save you from my rambling.
http://dailytriathlondiscipline.blogspot.com/2020/06/at-four-state-challenge-my-covid.html
I basically must run 23’ish miles while on a camping trip (at the starting point).
However … I come from pancake flat land which I assume I’ll not find (from where I need to start).
Just basically need a LONG out and back (from my starting point) hopefully trying to avoid any REALLY steep sections that will slow me to a walk / crawl.
Need to get a “quick†23 in for training purposes and then back to the fam. 
The north route might be the best bet. Both the Harpers Ferry and Brunswick routes have the Weaverton Cliff switchbacks in the middle which are not runnable at all. However, you have a good portion of the canal that would be smooth and flat. So if you don’t mind the vertical interruption, you could go with those routes.
Be prepared for some really rocky ground. North of the Weaverton Cliffs is pretty much a non-stop minefield of rocks to negotiate. All portions of the trail are a decent technical trail run.
Thanks very much!
Looking forward to it.
Appreciate your insight.
Have a great 2021!
Having ridden the GAP / C&O a few months ago, why not just drive to Harpers Ferry and run on the bike trail? Flat, good running surface. Unless you can’t steal the vehicle while you’re gone, this would be the easiest run.