i'd like to point out in the very beginning of walking dead, when rick and glenn cover themselves in guts while they're still in Atlanta, the zombies could at least jog. you see it when the rain comes and starts to wash the blood and guts off of rick and glenn. after that, yes they just shamble around.
my guess is nicotero would explain that in the early stages of zombification, if your body is mostly intact the zombie could at least walk at a brisk pace or slightly jog.
fast zombies have always freaked me out more, but it almost makes the antagonists (the zombies) too over powered for the story. look at world war Z, they act like ants climbing over that wall in jerusalem. i know it's a fantasy movie, but that seems a little far fetched.
the cancer zombies from I am Legend at least had a weakness. they couldnt go out in the sun.