An elderly woman walking with a cane on the Upper East Side was critically hurt Monday morning when she tripped and fell and was struck by a box truck whose driver then headed over the Ed Koch Queensboro Bridge, police said. The driver was heading east on E. 59th St. when he struck the woman near Second Ave. after she tripped and fell into the street about 10:10 a.m. just feet from the entrance to the Queens-bound side of the bridge. The driver continued on across the bridge to Queens. Medics rushed the victim, believed to be in her 80s, in critical condition to New York-Presbyterian Hospital Weill Cornell. It was not immediately clear if the driver, who is still being sought, realized he struck the woman.