The chasing, no matter how far, is really kinda wierd. But if there would be a max distance, you could just always run away, into the woods, until you are safe. Not sure what would be a good solution here. Maybe the chance of being knocked over could be incrased, so running away has its risks too ( i.e.: Zombie attack from the back while running has a 50% chance to make you fall down). Zombies should run faster than you in this case, or be able to attack you, while you are running. This isnt possible right now i believe?! And if you already got a distance and can run away, a Zombie should stop chasing you at some point.