Generally what I would believe is that zombies would have an
initial infection period where they could have full running
ability, however the biological system of a zombie is shut down
for the most part except for basic brain and muscle function.
Due to this the zombie may only have the ability to run for a
short time until being reduced to a slow stagger due to muscle
tearing. Remember zombies are necrotic. They are eroding. As an
unexpected result of this I’d guess that the zombies would
infect a large group of people at first which would infect more
people and then even more. So it would be terribly difficult not
to be killed within the first 96 hours of the outbreak (assuming
that it happened near you) due to the high functioning ability
during those key moments. If you’re still alive by the time the
zombies damage their muscle to a point where the can only do a
slow stagger toward you you’re out of extreme danger. But keep
your guard up, there still are hordes of undead just waiting to
feast on you! This