So Resident Evil 6 doesn’t come with any kind of book, and the tutorial is pretty vague and short.. and now you can’t remember how to run.
All you need to do to run is either hold the A (360) or X (PS3), or alternatively you can click and hold down on the left analog stick (like you would in the Modern Warfare games).
This isn’t a toggle however, you have to click it down and hold it to keep running.
Also remember that your stamina is limited, so don’t try to run forever, or you won’t even be able to fight off a zombie when you stop.