Aim <p>Sustainable initiatives which can multi-solve physical inactivity and social isolation&#xa0; are valuable approaches for improving population health. “parkrun” is a mass-participation free, weekly, timed 5&#xa0;km walk or run operating in 23 countries. Our study aims to test and extend previous qualitative research which claims that social connection is facilitated in parkrun, using a prospective social network analysis.</p> Subject and methods <p>We conducted a time dynamic analysis at two new Australian parkrun sites (one rural, one urban) across three waves over 12&#xa0;months using (1) social connections from a social network survey and (2) demographic, participation and performance metrics from parkrun administrative data.</p> Results <p>Network members (urban <i>n</i> = 354, rural <i>n</i> = 145) became more interconnected over time, especially at the rural parkrun. The network structure at both parkruns comprised a distinct socially connected core persisting over time despite turnover in members, alongside a less densely connected periphery. The core contained those with higher participation (walking, running, or volunteering) rather than those with faster parkrun finish times or better fitness scores. Women were disproportionately represented in the core at both parkruns, and a trend away from the core was observed for young adults over time.</p> Conclusion <p>Our findings suggest that more intense engagement with parkrun promotes social connection, regardless of location. However, the core–periphery social structure offers recurrent opportunity for connection while not requiring it. Attracting greater participation and volunteering among men and young adults would improve their social connectivity and the sustainability of parkrun.</p>