You will have to pay attention to the blue gauge located on the bottom-left corner of your screen. That's the stamina gauge, and the red one is your health gauge. When you run or do any other physical action, the blue gauge will quickly go down. As a result, you will have to wait a bit for it to recover. But if you don't want to wait and would rather just use this infinite stamina glitch, then simply press LB and tap it. You will notice that your character will keep moving without using your stamina at all.