Simulations of surface smoothing (healing) by Langevin dynamics in large systems are reported. The surface model is described by a two-dimensional discrete sine-Gordon (solid-on-solid) equation. We study how initially circular terraces decay in time for both zero and finite temperatures and we compare the results of our simulations with various analytical predictions. We then apply this knowledge to the smoothing of a rough surface obtained by heating an initially flat surface above the roughening temperature and then quenching it. We identify three regimes in terms of their time evolution, which we are able to associate with the resulting terrace morphology. The regimes consists of a short initial stage, during which small scale fluctuations disappear; an intermediate, longer time interval, when evolution can be understood in terms of terraces and their interaction; and a final situation in which almost all terraces have been suppressed. We discuss the implications of our results for modeling rough surfaces. Work at Los Alamos is performed under the auspices of the U.S. DOE. Work by A.S. was also supported by MEC (Spain)jFulbright at Los Alamos and by CICyT (Spain) through project MAT95-0325 at Leganes Publicado