Background: After-school programs (ASPs) have the potential to contribute to moderate-to-vigorous physical activity (MVPA), but there is limited empirical evidence to guide their development and implementation. Purpose: This study assessed the replication of an elementary school running program and identified psychological correlates of children's MVPA. Methods: We used pedometry to measure participant PA%, MVPA%, and MVPA% of PA during a 20-session alternating treatments design and examined associations between various psychological constructs and MVPA levels using odds ratios. Results: PA% (62.2% vs 76.1%, effect size [ES] = -0.65) was lower and MVPA% (33.3% vs 15.8%, ES = 0.75) and MVPA% of PA (53.6% vs 20.2%, ES = 0.91) were higher during game vs lap running conditions. The constructs of recognition, ego orientation, and expectancy beliefs distinguished between children with high and low MVPA. Discussion: The replication of results for MVPA with a different cohort provides evidence of program generalizability. Only game days met the ASP national recommendation for providing activities at an intensity of 50% MVPA% or greater. Translation to Health Education Practice: Running laps and running games both contribute to PA accrual, but they do so in different ways. ASP providers should weigh the demands and outcomes of each format.