Last week, we covered the move by the 15th Marine Expeditionary Unit, out of Camp Pendleton, Calif., to assist flood-ravaged Pakistan. The update: The situation is serious enough that Marines are now planning to assist for up to four months. Brig. Gen. David Berger, director of operations at Marine Corps headquarters, sat down with two other reporters and me yesterday to explain the situation. The amphibious assault ship Peleliu has been off the coast of Pakistan since Aug. 9, and more than 200 Marines are now based out of Ghazi air base, west of Islamabad, where helicopter relief missions are…