The morning clouds have broken, dispelling the threat of rain. I head over to Waterfront Park, where a clutch of joggers limbers up near the Burnside Bridge. On this Saturday morning, I have traded in my reporter's notepad for a tape recorder and a pair of running shoes. My mission: to infiltrate Run Against Bush, a grassroots organization dedicated to defeating George W. Bush in November by an unconventional method--putting one foot in front of the other. Started in February by an impromptu group of Washington, D.C., political neophytes, Run Against Bush now boasts 4,000 members from all 50 states...