Running. Why and how far?

It's all too easy to break in to a sweat these days. Just find a flat expanse, destination not required, apply some considerable effort and there you have it - running. Running can take many forms and requires little coherent thought. Even though there are those who may argue the opposite. Running on an overwrought brain surely cannot be good for your health. However running with imagination may lead to unexpected thoughts. Now there's an idea!