The issue of the last-mile in public transportation is crucial to its attractiveness and utilization, and thus represents one of the most significant challenges of the mobility transition in rural and urban areas. This contribution introduces a mathematical model represented as a directed and weighted graph to simulate public transit networks. The model is enhanced by incorporating edges that represent the last-mile connections between transit stops and final destinations, addressing a critical gap in accessibility. These edges are assigned weights that vary based on the travel resistance, as evaluated using the established methodology for the standardized evaluation of transport infrastructure investments. The methodology specifies a modal split proportion for public transit on a given route and computing the maximum allowable travel resistance as the objective function under specified constraints. This framework addresses the fundamental question of how the travel resistance of the last-mile should be in order to achieve a desired modal shift towards public transit.