Just about every Saturday morning, a group of baby boomers makes its way around a Springfield, Ill., park. They move fast, and there is a definite stride and determination, although they’re not out for a run. They pair up, peel off and circle the park again and again — taking short breaks at a water cooler — because the workout is far from easy. These baby boomers are racewalking, and they’re part of Abe’s Striders, a local racewalking club.