But the recent introduction of an all-in-one treadmill desk from Details may inch work-walking into the mainstream, as dozens of businesses invest in the hardware to let their employees walk (and, ideally, lose a little weight) at work.Since last November, about 335 Walkstations, have been sold nationwide to companies including Humana, Mutual of Omaha, GlaxoSmithKline and Best Buy.The Walkstation,