Picture this. You’re on a run, feeling unstoppable. Your legs are strong, exploding off the pavement with ease. The rhythm is perfect, and you think to yourself I could keep going forever. A mile later, it's a different story.  One moment you’re flying, the next you’re falling apart. Your legs turn to lead, your muscles are drained of their strength. You feel weak and lightheaded, like your body is wading through quicksand. Every step feels like a fight and the eff...