I only went out for a walk...

"I only went out for a walk, and finally concluded to stay out till sundown, for going out, I found, was really going in." ~John Muir

John Muir is simultaneously inspirational and enigmatic.  Part reclusive mountain man and part ecstatic holy man, his passion for the beauty of nature led him on incredible adventures, and his work and writings were the greatest inspiration for the creation of America's National Parks.

For more on Muir, read up at the Muir Exhibit at Sierra Club.
Otherwise, check out Part One of Ken Burns' magnificent documentary on our National Parks, America's Best Idea, called "The Scripture of Nature."

My question has to do with the quote above.  What did Muir mean by, "for going out, I found, was really going in?"