"Walter!" It comes out as a startled bark. Walter's the last person who should be out here, so of course he's out here and completely oblivious to the danger. Carrying a paper bag like a mother running after a schoolbus. "Damnit!"

Peter angles back toward the old man and grips him hard above the elbow, forcing him to run faster. Not fast enough, probably, so he steers them toward some promising cave formations. Maybe they can hide out in there until the worm loses interest.