Imagine you’re running a 5K for your favorite charity. It’s your first race and you’ve trained very seriously for it. There are butterflies in your stomach as you cross the starting line that shift into something closer to steel by the time you reach the last half mile. Your heart is thrumming, your energy is pulsing, and you pound out that last half mile, high on endorphins and success. As you lean over your knees at the finish line taking deep, rapid breathes, you feel a gratifying burn in your muscles and the tickle of sweat on your back.