Never been to Virginia myself but we've got the same thing happening here in Cornwall constantly. Big black cats spotted on the moors, usually dismissed straight away by anyone official. Sounds like what you're describing though - that size, the low slung walk, jet black coat.
Honestly I think these sightings are way more widespread than people realise and theres something to the idea that they follow certain landscape features. Ley lines, ancient tracks, that sort of thing. Could be worth checking if the Parkway follows any old routes or ridge lines that might form a pattern with other sightings in the area.
Did anyone manage to get a photo or were you just caught off guard? Would love to hear more details about exactly where on the Parkway this was.