Gellable treatment fluids may have their gel times modified by using a polymer mixture therein that contains two or more polymers having disparate average molecular weights. Methods for treating a subterranean formation using such gellable treatment fluids can comprise providing a gellable treatment fluid comprising a polymer mixture and a crosslinking agent, introducing the gellable treatment fluid into a subterranean formation, and allowing the gellable treatment fluid to form a gel in the subterranean formation. The polymer mixture comprises a base polymer and an addend polymer, where the base polymer and the addend polymer each comprise an acrylamide monomer unit or a derivative thereof and have different average molecular weights, such that a molecular weight distribution of the polymer mixture contains at least two local maxima.