We investigated qualitative features of an electron-transfer reaction in the case where the viscosity of a solvent is high and the activation barrier for the reaction is low so that decay of the reactant population is nonexponential. Using the reaction-diffusion equation for the Sumi-Marcus model and dividing a slow nuclear coordinate into diffusion-dominated and reaction-dominated regions, we derived expressions for the powers of dependence of the generalized mean lifetimes on the viscosity. We also derived an expression for the survival probability at short times. These expressions show fractional (not integer) powers and an algebraic decay at short times under certain conditions, which were found previously by numerical calculations. Moreover, from these expressions we found that the time dependence of the survival probability at short times is intimately related to the viscosity dependence of the mean lifetime. The expressions are in reasonable agreement with exact numerical calculations for large viscosity.