Interesting, and I would refine this by suggesting that the exercise is actually making use of the chemical changes induced by a stress response, which primes the body to react quickly and forcefully. Glycogen in the muscle is mobilized to glucose and blood flow to the muscles is increased. The body is ready for action, and going for a run, swim, etc uses these physiological changes as nature intended. And to your point, job done, threat gone, go back to normal.