Given the wording says parked, rather than left indicates it was actually parked out of the way.
But this is a small, 2 lane country road and there actually very few places on the south section of this road where it's viable to be "parked" off the road.
Somewhere like this where it's a small layby. There's only a few small layby I can see I think on the map from a quick scan of the road.
However further north towards Stanhope there's seemingly a section
where cars do park up, looks more like a potential hiking spot up that way and it's near to Tow Law.
*UPDATE* Just noticed the BBC article posted above which says the search is now focused on Hamsterley Forest which looks to be a good several kilometers away (east) from the road her car was parked on. Possibly they have learned of a particular regular running route she takes.