We have developed an analytic model for a preheated, polytropic IGM in hydrostatic equilibrium within a NFW potential in which the evolution of the halo structure between major merger events proceeds inside-out by accretion. For a polytropic index of 1.2, consistent with both many observational results and predictions from high-resolution gasdynamical simulations, our model is capable of reproducing faithfully all the observed X-ray properties of nearby relaxed, non-cooling flow groups and clusters of galaxies with a universal level of energy injection of about half a keV (see Solanes et al. 2005 for details).