Funny you should say that.

I left out my own anecdotal experience of walking 10+ miles on rural KY highways because I thought it might be too boring or personal. But since you asked...
A friend and I did this very thing, in KY no less (though farther north than Bardstown; we were in a rural part of Campbell County, south of Alexandria). We were of merely average fitness level; I had already started having health issues, and neither of us had lots of endurance. And we did it in 2.5 to 3 hours, TTBOMM. We were tired by the end, sure, but by no means destroyed. Our pace varied from moderate to brisk. We never jogged or ran, either. Just walked.
Dark? No, it wasn't dark that time I was out walking; but from other times of walking, I'm not sure how much that matters on a paved road. The road is the road, it's clear and blank, visible even in the dark; it's smooth, not rocky; you follow it, that's that, what else?
There aren't grizzly bears or any deadly wildlife round here. It's not even like other parts of the US in that you'll go lots of miles between houses or towns. You couldn't even die of thirst between one rural KY town and another. You'd find a farmhouse first... in most parts of KY, anyway, and certainly on a road like the Bluegrass Parkway.
To address another question: How many people saw us walking? Damn few, actually, as I recall. And this was on a Sunday, in the daytime, in a place where country folks still go for a Sunday drive as a special treat.
How many people might have seen whoever drove Crystal's car on a Friday night (or whenever), including her? No idea. I'll leave that question to the truly local folks who know traffic patterns. But I believe someone already answered your question, to this effect: it's quite possible no one would have seen her car, or anyone walking away from it, at all.
I have no idea who left her car there, or when. My only protest is against the idea that the car was somehow left an unwalkable distance from anywhere.
In my common sense and personal experience, since you asked, no, I do not believe her car was so unwalkably far that it would necessitate an accomplice. Nor would hitchhiking need to play a role. Or a moped, or a bike, or anything. The distance from that car to any place of any significance was
absolutely walkable.