I have always been a horror movie fan, and zombies of course have always been on the top of the list of horror. Zombies movies and games are something I have followed and had a huge interest in since I was a kid. Watching movies and playing games with zombies when I was younger all had them walking. My first zombie video game was Resident Evil and other then a few creatures in the game, all the zombies walked and were not fast at all. This is were I believe I started believing that zombies were just slow and could only walk, also go by a lot of early movies like Night Of The Living Dead.
Personally I don't really see how a rotting corpse could have the power to run at full speed all the time. I think that a zombie could maybe be fast when first turned since it would not be at that rotting stage yet. But a zombie that say came from the ground from already being dead, or that has been around for a long time would not have the muscles to just sprint and continue running non-stop. Even walking zombies still pose a huge threat. Well they might be slow, when a hoard of them are coming down on you, it would be very difficult to escape. You also have to remember that the slower they move, the quieter they will be. Walking zombies could see someone from a distance and not making any noises fellow them with out that person even knowing. You could lead a zombie right to your camp, and it would not be fun to wake up to a zombie biting you or a fellow survivor.
Walking would also be less obvious of being a zombie since they are just walking around making no noises looking for food. You could walk up on one thinking it was someone hurt or just lost until your being attacked because you were caught off guard. At the same time though, a walker would be easier to kill cause they are not moving very fast. This would make head shots even easy for people with no sniping skills or even give the option to walk up to one and shot it at close rang given there was only a few of them not a hoard.
When I think would could make a zombie be able to be a runner, I think that it must be because it has become feral. Out of its mind and wild, as runners appear to be, it pushes through its body rotting and just goes. We all know that zombies don't feel pain and just keep coming until they are killed. So seeing its next victim would just make it go wild and push though anything on its dying body to get to its meal. Now like I said, I don't think a zombie that has been around for lets say a year, would not be able to run anymore.
Runners would be more dangerous because of there speed. And since there are no real zombies attacks yet, I'll go off what movies have shown and say that they would make more noise with yells and by snarling. They would also be hard to kill since they are fast and will be contently moving. At the same time they would give themselves away by all the noise they make. They chance of a runner sneaking up on you is less likely.
I like the concept of the zombie in the video game Dead Island. There are a bunch of different kinds of zombies in the game, but I'm just focusing on the basic two types of runners and walkers. In the game they can do both, but they have it make sense. The ones that run are titled as infected. Meaning that they are in between still being human and becoming a zombie. Not yet decaying and not human anymore, they have turned rabid and attack the living with furry. Swing there hand wildly and not really trying to kill for food, but just cause there going out of there minds. Once the infected turn fully into zombies, or walkers, they become slow. They will pick up a little speed when trying to get to the living, but for the most part walk slowly around or just stand in one spot. They have started to decay and when they find some prey go straight for a bite not caring about nothing but eating.
My beliefs or thoughts on zombies are just that. I personally don't know if there was a zombie apocalypse if there would be running or walking zombies. Both are scary in there own right. Runners coming right at you at top speed never running out of breath, tearing at you violently as they eat away. Walkers coming slowly towards you as you shoot at them with no effect, taking there time ripping you open and eating you very slow.
Like I said, if anything when talking about runners and walkers, I think the video game Dead Island got it down. Once they are complete zombies, they will start to decay and would not be that fast. But that in between stage of being infected would allow them to be runners. Personally I hope this question will also be a debate and we will never have a zombie apocalypse and find out if the zombies will be runners or walkers.