Subject to valid existing rights, each wilderness area designated by section 460vv–1 of this title shall be administered by the Secretary of Agriculture in accordance with the provisions of the Wilderness Act of 1964 [16 U.S.C. 1131 et seq.] governing areas designated by that Act as wilderness areas, except that with respect to any area designated in section 460vv–1 of this title, any reference in such provisions to the effective date of the Wilderness Act of 1964 shall be deemed to be a reference to the effective date of this subchapter.