Hole transport in molecularly doped polymers (MDPs) is modeled as random walks on fixed donors (Ds) embedded in a polymer matrix. Dilution p<1 corresponds to placing individual Ds, dimers D-2, or tetramers D-4 randomly on a fraction p of sites in a face-centered-cubic lattice. Monte Carlo simulations of the drift velocity v(D)(E) in a bias field E have maxima in dilute (p=8%) systems of D-2 or D-4 that are related to the formation and polarization of clusters of nearest-neighbor donors. Marcus or small-polaron hopping rates with fixed parameters account for the concentration, field, and temperature dependencies of the mobility, mu(E,T)=v(D)(E,T)/E, of D=TTA (tritolylamine) in polystyrene and of related systems with D-2 or D-4 in PS. The compensation temperature is lower for D-2 than for D at p=20%, consistent with stronger positional disorder for dimers. The anomalous broadening of photocurrents in D-4 with increasing E is due to cluster polarization. The parameter sigma=700 K for energetic disorder is used throughout and is similar to25% smaller than in the Gaussian disorder model. Spatially correlated energies yield the characteristic field dependence of mu(E,T). Although not quantitative, the comprehensive treatment of dilution in TTA:PS and related MDPs clearly supports Marcus hopping rates and stronger geometrical than energetic disorder.