The treadmill sat in the corner of my garage for three months before I finally worked up the courage to turn it on. I remember the day clearly. It was mid-October, and the rain had been hammering our roof for five straight days. Cooper, my ninety-pound Golden Retriever, had been pacing the living room like a caged animal, his nails clicking against the hardwood in that frantic rhythm that always made my husband laugh. "Look at him," Mark had said last Tuesday, watching Cooper circle the coffee table for the thirtieth time. "That dog needs to run. Like, really run. And I'm not standing in the rain again." Neither was I. Not after last week's disaster, when I'd come home looking like a drowned rat, mud splattered up to my knees, while Cooper had rolled in something dead and then shook it all over my brand new coat. So I'd bought the treadmill. Secondhand from a woman on Facebook who swore her Border Collie used it daily. It was industrial-looki...