Jenny Bruso didn't expect this. When she went on her second-ever hiking excursion on her 30th birthday and a first date, she hated exercising. "At the time, it was one of the hardest things I'd ever done. I resent the way exercise is marketed in our culture, especially to women, as a way to fix our bodies—implying that we shouldn’t like ourselves the way that we are. But that day, I felt like something new opened up to me." Bruso began setting out...