Positronium (Ps) is simulated using Path Integral Monte Carlo (PIMC). This method can reproduce the results of previous simple theories in which a single quantum particle is used to represent Ps within an idealized pore. In addition, our calculations treat the e(-) and e(+) of Ps exactly and realistically model interactions with solid atoms, thereby correcting and extending the simpler theory. We study the pick-off lifetime of o-Ps and the internal contact density, kappa, which controls the self-annihilation behavior, for Ps in model voids (spherical pores), defects in a solid (argon), and microporous solids (zeolites).