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.