you can't be serious, if you remember the official 1-3 part gameplay videos released almost 6 months ago, in the beginning of the first part you can see Todd switching between "jogging" and walking. And he was playing with an xbox 360 non the less
That's called an anolog stick and the different speeds are dependent on the amount you push the stick.
Unless you use a gamepad, mouse and keyboard have no anolog input.