What makes an endurance athlete quit? Not quit the sport, but quit during a competition.  Every runner, swimmer, or cyclist starts a race with the desire to win or at least achieve a personal best time.  They’ve done the pre-race math - keep at a certain pace for the entire distance to achieve the target time.  Their wearable technology keeps them updated on heart rate, distance and split times to stay on that pace.  

However, at the finish line, many athletes are not able to maintain their strides/strokes per minute, giving in to the perception that their energy tank is empty.