We study a system of neutral two-level atoms prepared in a collective state coupled to an optical waveguide, particularly considering the regime where interatomic separations are comparable to the coherence length of a spontaneously emitted photon in the waveguide mode. Considering the atomic system interacting with the quantized electromagnetic field bath, we find that there is a departure from the Markovian dynamics due to the retardation effects in the field propagation and backaction of the bath on the system. We find that the non-Markovian dynamics in such parameter regimes can exhibit several interesting effects such as a time-delayed feedback-induced enhancement and inhibition of the collective spontaneous emission in addition to the usual Dicke super- and sub-radiance. We further discuss the feasibility of observing these effects in ongoing experiments with atoms coupled to waveguides.