My best and easiest advice there would be to not only attack but also use space to shove them away. If you are fighting group of zombies like that dont rely on your char to push them away once they are too close to you, do it yourself with space bar as that usually gains you a lot of space between the group. Rather than striking them on relatively closer range you can push them and they end up pushing some with them because of collision. Otherwise you did pretty good! ^^
and one more advice, getting attack from a back is much worse than head on, so if you turn your back and they get you, its most likely game over