Their first and only date was an innocent stroll through Central Park. But when an Upper East Side woman rejected her running-club pal’s bid for an encore, the creep lost his mind and embarked on an 18-year reign of terror — stalking her through six states as she desperately tried to escape his reach. “I was terrified of him,” Linda Arnaud, 45, told The Post of twisted Queens native Moses Shepard, who last week was finally found guilty of federal stalking charges. “He showed up at my place of work. He showed up at my school, at my apartment —...