If you run forward and running away from zombies, the zombies are much too fast. In order to accurately say that zombies are faster than the player, what we also have to see that the zombie runs two steps and then behind the player is left, runs again, stand back remains ...
The reason lies the zombie until it is sometime slow, always in the back against the player.
And if you run as fast backward as forward, then you will be defeated permanently by zombies. There are probably at the stroke system of zombies an error between walking backwards and forward walking and face the zombies back (beaten) and the back of the Zombies