Adopt the 20-minute rule. If you're not sure whether your body is telling you to take a day off or plow ahead, see how you feel after 20 minutes. 'Go out the door always, says coach and 2:13 marathoner Brad Hudson. 'If the pain is worse or the same, take a day off; if it's better and you feel nothing, then do the workout.' Twenty minutes is long enough to shake off inertia that plagues all of us from time to time, but not so long that it will worsen an impending injury.