There are various kinds of zombies in this game. Some are faster than others.
If you open up monsters.json you can note the speed of each. Your speed (unencumbered) is generally 100, so you can measure your speed against the speed of the various zombie types.
The generic zombie that you see the most of has a speed of 70 and you can outrun them, but a spitter or a shocker, for example, are slightly faster than you and will run you down.