I always liked the Romero Zombies: slow but hard to stop, always in large numbers. Killed only by a shot in the head. I think high speed runing is silly mostly because zombie is really a dead body - walking. THats why you call them "the Walking Dead" and not "the Running Dead". Rotten, disfunctioned body that is animated by the virus or whatever jest to keep walking (and eating brains of course). On the others side - gameplay wise - it will be just too easy if you could get in, get out of a city just by running fast. The truth is - only on a diffirent (urban) terrain slow zombies would be ok. Labirynths of small streets, dead ends, lots of corners PLUS lots of zombies witch are only killed by headshot. On an open terrain it would jest be too easy to avoid them. THats why most of zombie movies are set in cities/towns/malls ;) and not in woodlands and beaches ;)