For Mary Heffernan, being an entrepreneur is a lifestyle, one that demands complete attention and commitment—and, sometimes, a willingness to sleep on the floor. At age 44, she and her husband, Brian, run Five Marys Farms, a ranch in Siskiyou County, California, with free-range, pasture-raised Black Angus cattle, Berkshire hogs, and Navajo-Churro sheep. They also…