True toughness comes from overcoming personal limits. It's fighting against the internal dialogue in your head that stems from your body wanting to slow you down. It's overcoming the feedback signals from your muscles that say they're flooding with acid, or that your fuel stores are running dangerously low. How each runner experiences these sensations of fatigue is completely individual. No one else knows exactly what you're feeling and at what point you might give in to the pain. Toughness is knowing when you've gone past previous barriers and tangled just an instant longer with your brain as it tries to shut you down.