By T.C. Lumbar As bikemates 20 years his junior bowed out with sore legs and stiff muscles, Gus Rivera was proudly the last man standing. A year and a half earlier, and 75 pounds heavier, Rivera, 57, didn't even own a bike. At the time, he had begun a weight loss journey, shedding his first few pounds by walking. But progress was slow. "It just wasn't working as much as I wanted it to," he says. Eager to try something new, and unable to run because of joint pain, he hopped on his first bike since...