Just wanted to update that I did end up calling this in (especially after reading more about Belt Parkway access, and verifying exactly where I was and when through MapMyRun). They didn't sound super interested (obviously) but said it'll be passed along in case they send a patrol out there to see who might be hanging out in those marshes as well. I also read up on the efforts to clean up the overgrowth on Howard Beach and add cameras/police presence and I really hope it'll come to pass.
I still haven't quite felt back to "normal" ever since this happened, I guess because of the distance to me and the overlap of circumstance. Last night I had a nightmare about somebody waiting in the woods for another young woman - anybody at all - and it was kind of that "running in slow motion" dream feeling where I couldn't do anything with this information to help anybody, it was just weighing in my gut that somebody had chosen a wooded trail at random, very close to its exit/entrance, broad daylight again. And then I woke up to news about Vanessa Marcotte and I just feel sick. IMO they're not related, save for the possibility of some psychopathic loser being inspired by the coverage on Karina's case.