The topics of serious leisure and subjective well-being have been discussed extensively in previous research. It is generally acknowledged that people prefer to experience deeper satisfaction and happiness through serious participation in leisure-time physical activities. However, it is essential to examine the relationship between serious leisure and subjective well-being in an urban setting as well as the mediating effect of leisure satisfaction. Data were collected from 447 recreational runners at the 2018 Wuxi International Marathon event in China. The study results showed that serious leisure was positively associated with leisure satisfaction and subjective well-being, that leisure satisfaction was positively associated with subjective well-being, and that leisure satisfaction completely mediated the relationship between serious leisure and subjective well-being. Running group membership significantly affected the path from serious leisure to leisure satisfaction, while other demographic variables (e.g., gender and education) did not moderate any paths. These results help explain the intricate relationship between serious leisure and subjective well-being and offer theoretical and managerial implications for serious leisure.