Introduction Current deficiencies in children’s leisure-time physical activity in the United States and globally underscore a pressing and ongoing need to enhance child ecosystems that include school and out-of-school time settings with physical activity supports and opportunities. Marathon Kids (MK) is an international nonprofit organization that promotes children’s physical activity via school and community-based running clubs and programs. Building from the importance of indigenous knowledge and colearning with stakeholders within real-world settings for children’s physical activity promotion, we explore best practices for the implementation of children’s running clubs based on a national sample of U.S. adult volunteer MK coaches. As a secondary aim, we describe selected implementation characteristics of MK running clubs as relates to when clubs are implemented and average weekly minutes scheduled for running clubs. Methods This study was guided primarily by a phenomenological approach to qualitative research. Data were collected in 2019 from MK coaches via an online survey that included open and closed-ended items on MK delivery and best practices. Qualitative analyses were based on thematic analysis; quantitative analyses were based primarily on descriptive statistics. Results MK coaches (n = 478 out of 676 coaches; 70.7% response) from across the United States (n = 35 states) participated. Eight primary themes encapsulated the best practice recommendations: running club leadership and support network; general organization and planning; student recruitment; running club implementation; tracking/logging miles; social support; communication and public promotion of running clubs; and guiding principles for running clubs. MK coaches reported an average of 112 weekly minutes scheduled for running/walking. Conclusions MK coach best practice recommendations hold promise to enhance and further disseminate the establishment of volunteer-led running clubs for young people.