I really feel like that runner coming out of that same entrance where Katie was found around 1245 AM (do I have the correct time?) must have seen or heard something, even if at the time, he had no idea it was something concerning, much less a murder. If she is in the crosswalk 12:10-12:15 and her girlfriend finds her just after 1, I personally don't see a way she wasn't already killed by 12:45 or at least the attack was underway. It doesn't sound like she and her dog were only stabbed once. "Mutilated" and "brutal" have been used, as well as it being described as unusual and not something that ever happens here, so I am assuming (maybe correctly, maybe incorrectly) that she and/or her dog were stabbed multiple times, which would take a bit of time. Even if let's say the attack hadn't started yet when the runner came through, she was likely in the vicinity and very likely her killer, too.
I am not suggesting the runner had anything to do with it. And I'm not suggesting he witnessed a murder and kept it quiet. Perhaps he only saw the dog, like someone else brought up, and thought "That's odd," but just thought it was a sick dog and wanted to stay/get away from it. If Katie was off to the side, maybe in shadows or what not, he may have not noticed her. Can we tell if he has earbuds in? That could account for not hearing anything, especially if they are noise-cancelling (I personally wouldn't go running at night - or at all, really - with noise-cancelling turned on, but that's a choice some might make).
It sounds like this murder is pretty well publicized locally. I hope he especially comes forward soon (and the others on camera, too).
Is this area of town more for "locals"? I guess I'm thinking maybe he has no idea there was a murder there or that he is wanted for questions if he was a tourist or a person in town on business. Maybe unusual for a tourist or visitor to run through a park around 1 AM, but maybe not if they are insomniacs or just used to running late at night.