Walking facilitates a quieting of one’s inner discourse in a way similar to meditation. It allows one to get out of ones head, as it were, and focus on the surroundings or just the simple, rhythmic act of putting one foot in front of the other and breathing.
In fact, there’s a lot of crossover between the psychological benefits of walking and the physical benefits of walking. Many of the mental benefits accrue as a result of the physical benefits and vice versa.
For example, walking improves sleep by helping work off energy that gets bottled up during the day for anyone whose daily routine involves sitting at a desk. Walking can release endorphins and lower cortisol (the stress hormone), which also improves sleep. Improved sleep, in turn, further lowers cortisol; a virtuous cycle, byproducts of which include both weight loss and a sharper, calmer mind.