They have stamina for sprinting in WarZ which is a feature I like, because as you said, the endless running gets old. I don't want "28 weeks later" zombies which are ALL of them, but I have unlimited sprint. A nice sprinter classed zombie to scare people like a witch in L4D. Know what I mean man?

Yes i've heard that there are some that will move faster than others, but they will still be slower than the player.
Kinda hard to picture since i haven't seen it, mostly all the Zombies i've seen in the videos moved fairly slow.
But from my knowledge they haven't shown off the stem cell zombies yet and i can see them as being faster and harder to kill since they give out the stem cells that can be exchanged for currency.

It was mainly the way in DayZ that they looked like they came at you full sprint and how once you entered a building they slowed to a crawl.
to me it wasn't scary at all and was a big break in the Sim environment that it was trying to achieve.