You assume right, BB! I think fast zombies are really interesting but survival just wouldn't be feasible. If they had speed on top of their vast numbers, they'd be able to catch you too easily in my opinion. They're not runners but they won't be too slow either. Their max speed would be equivalent to that of a slow jog but since there's so many of them, they're still very dangerous.

As for the question above, are you actually being serious? If so: suspend your disbelief. There are innumerable scientific reasons that zombies either cannot exist or would only last a few weeks before succumbing to extraneous factors such as putrefaction caused by warmer climates, muscular rigidity from colder climates, acclimated damage left unhealed due to the lack of an adequately functioning central nervous system and many, many more...

... but that wouldn't make for a very interesting RP.