We present a simple coarse-grained model of silica sol-gel materials which is applied to investigate the formation and structural and mechanical properties of low-density aerogels. This model, unlike many previous developments, is fully flexible and can account for bond breaking and creation upon deformation of the gel structure. We show that direct comparison with experimental structures can be made for appropriate choice of bond-formation control parameters, and that quantitative results for bulk modulus and other properties can be obtained. Additional simulations of syneresis, compression, and extension are performed and microscopically analyzed in order to correlate macroscale behavior with local gel structure and its evolution. The dependence of gel structure and mechanical properties on both "experimental" variables such as reactivity, bond strength, and viscosity, and "simulation" variables such as cell size and integration method, are evaluated. Finally, gas adsorption in fully flexible gels is considered through coupling of a mean-field lattice gas model to the coarse-grained gel model via a perturbation-theory-like approach.