Research suggests that exercise groups can provide social resources, such as a social identity, social support, and social comparison, and that people may join groups to manage uncertainty during times of change, such as midlife. This study examines how midlife adults utilize Peloton, an online group exercise cycling platform, as they navigate midlife. Twelve Peloton participants aged 40-64 underwent three data collection stages: (1) an initial exercise history semi-structured interview; (2) a two-week post-exercise diary; and (3) a follow-up topic exploration interview. Data were analyzed using abductive thematic analysis informed by the social identity approach. Findings reveal a midlife process of identity erosion, characterized by reduced agency and physical decline, prompting a reassertion of competitiveness through online exercise and social comparison. This study extends the Social Identity Model of Identity Change into transitional non-event-based (e.g., life stage) rather than event-based (e.g., retirement) life changes. It suggests that identity erosion precedes identity change in transitional life contexts, and that identity remooring may occur sequentially. This study suggests that live feedback and real-time interactions facilitate the formation of group norms and social identity in online exercise groups, even in the absence of live communication.