We present a study of the relaxation dynamics of the photoexcited conductivity of the impurity states in the low-density electronic glass, phosphorous-doped silicon Si: P. Using subband gap optical pump-terahertz probe spectroscopy we find strongly temperature- and fluence-dependent glassy power-law relaxation occurring over subnanosecond time scales. Such behavior is in contrast to the much longer time scales found in higher electron density glassy systems. We also find evidence for both multiparticle relaxation mechanisms and/or coupling to electronic collective modes and a low temperature quantum relaxational regime.