A spectroscopic investigation of bound electron-hole pairs in GaAs propagating with large center-of-mass momentum is presented. The approach is based on transmission experiments exploiting the coupling of excitonic polarization to the electromagnetic field of femtosecond laser pulses. The dispersion relations of the coherent excitations are determined up to excess energies of 300 meV above the band edge. A surprisingly low Frohlich coupling of light-hole excitons is observed in excellent agreement with theoretical simulations. The influence of different phonon scattering processes on the excitonic damping is discussed. The interaction dynamics of excitonic wave packets with nonthermal carrier distributions is analyzed with femtosecond time resolution. A theoretical model based on the exciton-polariton concept including additional boundary conditions reproduces our experimental results. The observations. in extremely thin samples show deviations from our phenomenological model, This finding is important for a detailed understanding of the microscopic polarizability near semiconductor surfaces.