What the Pros Do: Just because pros may be on a structured training plan, doesn't mean they are inflexible in their training. If they are feeling tired or simply having an off day, they may look at their wattage and their heart rate on the bike and adjust their workout accordingly.
What You Can Do: Listen to your body. Missing—or making adjustments to—one workout won't make or break an entire season, and responding to how you feel will allow you to feel properly rested and go faster the next time you race.