"We Want parkrun to Well Outlive Us": Behind the Dissemination and Sustainability of parkrun in Australia.

parkrun is a free, weekly organized mass participation physical activity event providing a remarkable example of at-scale dissemination, with over 1 million participants and 150,000 volunteers in Australia. This qualitative study draws on stakeholders' experiences to explore how parkrun succeeded with its dissemination and sustainability in Australia. Maximum variation and snowball sampling methods were used to select interviewees representing three parkrun stakeholder groups: parkrun Australia employees; volunteer organizers (event directors and ambassadors); and local external stakeholders (e.g., running clubs and landowners). Semi-structured interviews were conducted with 67 adult stakeholders in two phases (February-May and August-October 2023). We conducted a reflexive thematic analysis of interview data to explore contextual factors and mechanisms of parkrun growth and sustainability. We identify four themes to explain the growth and sustainability of parkrun in Australia: "Belief in an essential parkrun" comprises varied but overlapping ideas of what parkrun is fundamentally about; "Fluidity of movement" reflects the way individuals move freely in and out of the parkrun model, between roles and events; "Organizational and individual evolution" describes individuals' engagement journey and how the organization adapted with growth; and "Shared custodianship" reflects the dispersed leadership and shared responsibility across time, place, and people. These themes illustrate elements of the parkrun model that create broad buy-in, foster stakeholder commitment and longevity, and maximize resources for enhanced reach, dissemination, and sustainability. Our findings contribute practice-based evidence that may inform scale-up and sustainment of similar public health interventions.