Yoga talks about prana. Certain Chinese martial arts and practices refer to qi. These are both, I believe, forms of energy, likened to flowing rivers, and sometimes they become blocked and stagnant (I have no doubt that should I be wrong about this, Teh Interwebz will correct me), resulting in poor health in the individual. Despite being a self-confessed hippy twat, this is a bit woo-woo even for me. However, I can't deny that there comes a point in recovery from illness where it just feels better to move, particularly outside.
To my mind it's far more likely to be something to do with increased circulation flushing out waste from the body's systems, rather than a mystical energy that no-one has been able to measure. To a certain extent though, it doesn't really matter what the explanation is if it works, and work it does.
Ten miles, super easy.