This paper presents an extended coarse-grained investigation into the elastic properties ofpolystyrene. In particular, we employ the well-known MARTINI force field and its modifications to achieve molecular dynamics simulations at the μs timescale, which take slow relaxation processes of polystyrene into account, such that the simulations permit analyzing the bulk modulus, the shear modulus, and the Poisson ratio. These elastic properties are used to gauge a promising protocol for calculation of various mechanical properties of a polymer system, based on the analysis of internal pressure in the system. Through modification of MARTINI force field parameters we elucidate that for various sets of polystyrene interactions the internal pressure of the system tends to saturate quickly enough to permit μs-long simulations suficient to predict elastic moduli close to those values reported in the experiment. We demonstrate that the suggested approach yields significantly more accurate results than the alternative analysis of internal energy of the system, and the performed analysis reveals that significantly longer simulations are necessary for a similar analysis in that case.