Hit the AT head either outside of Front Royal, Va on Rt 522 and go east to Mosby's Shelter or you can hit the head on Rt.55 outside of Linden, Va and it will take you to the same place.
Better yet is to take two vehicles, park one where you want to go in and one where you want to come out. I suggest going in at Linden, camping at Mosby's Shelter, and coming out at Front Royal.
At one time there was a large antebellum home there (barely a foundation left now) that was used as a hospital after the local Battle of Manassas Gap (Wapping Heights). It was also used as a "safe house" by Mosby's men when they raided along the Manassas Gap RR and that is how it got it's name.
There is a slave graveyard 100 yards north of the spring where both Union and Confederate soldiers that died of their wounds were buried. It's all grown over now but in the winter you can see some of the head and foot stones made of local rock slabs.
In the 70s (before it was logged and many were knocked over) I counted over 70 graves. Of course I can't say how many were slaves and how many were soldiers.
http://www.topoquest.com/place-detail.php?id=1470773
You can see how Mosby's Shelter is sorta halfway between Rt. 55 and Rt. 522
http://virginia.hometownlocator.com/maps/feature-map,ftc,2,fid,1470773,n,mosby%20shelter.cfm