When a traveler asked Wordsworth’s servant to show him her master’s study, she answered "Here is his library, but his study is out of doors." 
In an age of 24-hour gyms, extreme sports, and obsessive running and biking, it seems the humble art of walking outdoors has sauntered away from center stage. Perhaps our progressive, super modern age has rendered walking (as a calisthenic) superfluous because it seems monotonous but some of the most creative geniuses in history did not think so.
I have been reading Mason Currey's Daily Rituals: How Artists Work (you can read my highlights here) and I was struck by how many similarities there were in the artists lives, namely in uniformity, orderliness, and ... walking:
- Balzac took a walk, had a bath, and received visitors until 6: 00, when the cycle started all over again.
- After lunch he (Hugo) embarked on a two-hour walk or performed a series of strenuous exercises on the beach.
- Promptly at 2: 00, Dickens left his desk for a vigorous three-hour walk through the countryside or the streets of London, continuing to think of his story and, as he described it, “searching for some pictures I wanted to build upon".
- Darwin maintained a quiet, monkish life at Down House, with his day structured around a few concentrated bursts of work, broken up by set periods of walking, napping, reading, and letter writing.
- At sunset he (Hawthorne) went for a long walk, from which he returned late in the evening to eat a bowl of chocolate crumbled thick with bread ...
- Before dinner, Tolstoy would go for a walk or a ride, often to supervise some work on the estate grounds.
- After lunch he (Tchaikovsky) went for a long walk, regardless of the weather.
- Most days she (Georgia O'Keeffe) took a half-hour walk in the early morning, keeping an eye out for rattlesnakes on her property, which she would kill with her walking stick...
- I have one free hour and then an hour’s walk. Then 2 hours’ practice, and then I retire with the chickens. - Rachmaninoff
- In the late morning or just after lunch, Balthus would head to his studio on the outskirts of the nearby village— walking with the aid of a cane or, later, being pushed in a wheelchair by his wife, Setsuko.
- For me the morning is the best time to write. During the other hours of the day I attend to my housekeeping, take walks in Central Park ... - Willa Cather
- She (Carson McCullers) typically worked until the middle of the afternoon, then went for a long walk.
- When he got stuck, Barthelme would head out for a twenty or thirty-minute walk in the neighborhood.
- There are things which help me get in the mood to work. Cleaning for one. Ironing is great. Taking a walk is always inspiring. - Maira Kalman
- Then he (Georges Simenon) would go for a long walk, eat lunch at 12: 30, and take a one-hour nap. In the afternoon he spent time with his children and took another walk before dinner ...
As a runner who intersperses running with walking (and whose languid pace too frequently resembles even crawling at times) I can identify with these giants of the mind and spirit at least in their preponderance toward pondering. The physical repetition of walking is a natural complement to the psychical act of rumination (to "turn over in the mind"). Though they might have stepped away from their pens, their brushes, and their compositions, the mind in its circulatory perambulation wheels its way, most often subconsciously, around (largely) current affairs. Even when not at their work, seemingly unrelated activities such as walking feed the work. Movement suggests progress even if we are beguiled by our own lack of a real destination and though it's certainly anecdotal, this shared characteristic of so many imaginative people lends credibility to the suggestion that walking actually fuels creativity.
Walking as an exercise has even been proven to be as effective as running (but let's not tell the runners and the Cross-Fit crazies; leave them to their evolutionary superiority, their dumbells, and circular reasoning). Walking predisposes the walker toward redemption of the true meaning of the word 'calisthenic': beauty and strength.
If you would get exercise, go in search of the springs of life. Think of a man's swinging dumbbells for his health, when those springs are bubbling up in far-off pastures unsought by him! 
More than mere physical movement, walking outdoors renders the simple act near orphic in its power. I have written elsewhere that I started to enjoy my running when I learned to slow down and appreciate the world around me. A flower, a hill, reflections off the mirrored surface of a pond, a covey of birds startled to flight, a single scissortail preening its feathers on a fencepost, they each contain worlds of wonderment that (in my haste) were ignored.
Wendell Berry in It All Turns on Affection: The Jefferson Lecture and Other Essays gently rebukes our incessantly active lives and our blindness toward nature, obscured by our technological conveniences:
I have a growing instinct to avoid mechanical distractions and screens because I want to be in the presence of this place. I like to write by the ambient daylight because I don’t want to miss it. As I grow older, I grieve over every moment I’m gone from this place, because it is inexhaustibly interesting to me. Unexpected wonders happen, not on schedule, or when you expect or want them to happen, but if you keep hanging around, they do happen. When I’m up in my writing place, which is a very small building on the riverbank, I’m making no noise. If a flock of wild turkeys gathers around that little building and under it, I hear them. Since I don’t have a screen in front of me, I see them. Or I may see otters playing in the river.
Annie Dillard wrote similarly, "nature is very much a now you don't see me, now you do affair". Walking forces contact with a world beyond our limited networks, a world we can neither manipulate nor contain, whose brightness and vividness enrapture us (if we let it). Perhaps this was part of the secret of those early artists who first painted en plein air ("in the open air") in the early days of impressionism. "One of them, Camille Carot, the most talented and versatile landscapist in France, regarded plein-air studies as essential for capturing the fugitive effects of light and color".  It seems artists escape outdoors for both release and appropriation, bottling an essential beauty, illusive but for movement.
Rather than living vicariously through the adventures of others (via social media), perhaps we should pursue our own adventures, regardless of how undramatic and benign they may seem. Thoreau, in his famous essay on walking, intimates that we might be surprised to discover that no matter how small our escapades become, they may suffice for true adventure all the same. And, who knows? Even our beloved Bilbo Baggins reminds us that between "there and back again" lies a possible quest for far-wandering hobbits secure in the fact that unless we venture outdoors, we'll never experience the unique pleasure of knowing that not all who wander are lost.
Two or three hours' walking will carry me to as strange a country as I expect ever to see. A single farmhouse which I had not seen before is sometimes as good as the dominions of the King of Dahomey. There is in fact a sort of harmony discoverable between the capabilities of the landscape within a circle of ten miles' radius, or the limits of an afternoon walk, and the threescore years and ten of human life. It will never become quite familiar to you. 
 Walking, Henry David Thoreau
 The Judgment of Paris, Ross King