How far are those houses from the entrance Karina used? Did she loop around and exit that way too? Because if someone was watching her from one of those houses and saw her enter, is it likely that the Watcher could go to the entrance and walk into the trail, waiting for her to loop back as she always did?
I'm not sure why...but this seems personal to me. One problem with social media, IMO, is that it allows strangers to feel they know you. I wonder if her Instagram was open to the public. She posted beautiful artistic pictures, but I can see some stranger who knew her pattern of running...watching her...obsessing, fantasizing over her Instagram. And then, realizing, that he could make his twisted fantasy happen.