A realistic take on the common zombie (revised)

Over the years we all had argued what kind of infected would emerge if
there was ever a Z.A.  however, I believe that neither the slow
shamblers or the relentless runners would come out alone.

instead I believe that the infected would be a matter of both.

To elaborate:

When someone becomes infected, dies, then reanimates, they would be "fresh"
zombies, with half their motor functions such as running and climbing. 
However, after this Fresh zombies sits for a couple of weeks the zombie's
body will start to decay as a dead body should.

which, on top of non-vital body damage received during encounters with survivors, this
fresh zombie will start to lose several basic motor functions and in
time become the more classic shuffler, with no coronation, no way in
climbing up, and no way their rotting legs can run anymore.

In short, the rotted or "aged" a zombie is, the less lethal it becomes as it'll lose it's ablity to run and climb and so on.

of course even a shambler would still be a threat to humans as even a bite from one of these would create a fresh zombie from the recently killed survivor; not to mention they are attracted to sound; so with sound and their common want for our flesh, they will bunch up into a mob or horde to go to the direction of the noise.

now, When you think of it, if there were a ZA I would think the infected would turn into something along these lines. 
which, I would think would create a constant challenge for even the more
knowledgeable survivors on here.

As for how large scale cities would fall to this, keep this in mind, its a plague or virus that will spread like an invisible wild fire.