Objectives: This study reports on the long-term effects of the Better Life After Cancer: Energy, Strength, and Support (BLESS) program, a 12-week social capital-based exercise adherence program for breast cancer survivors (BCS), implemented using a randomized controlled trial design. The study investigated outcomes related to cancer-related fatigue (CRF), quality of life (QOL), physical activity, depression, anxiety, sleep quality, and social capital. Methods: Participants who had moderate or greater CRF were randomly assigned to the intervention (n = 24), consisting of supervised and home-based exercise, or the control (n = 26), who received exercise leaflets. Generalized estimating equations models were fitted for the outcome variables. The assessment points were baseline (M1), immediately after completing the intervention at 12 weeks (M2), 1 month (M3), and 6 months post-intervention (M4). Results: A significant reduction in the total CRF score was found for both groups. We observed a significant time by group effect at M2, indicating a reduction of behavioral/severity CRF scores and a higher increase of physical activity. Also, there was an increase in the QOL score of both groups at M2, M3, and M4, compared to M1. Both groups had reduced anxiety at M3 and M4 compared to M1. The time by group effect for depression, sleep quality and social capital was not statistically significant. Conclusion: This 12-week exercise adherence program improved behavioral/severity CRF and physical activity post-intervention. Both the experimental group and control group showed significant improvements in CRF, QOL, and anxiety domains compared to the baseline, which extended to 6 months post-intervention. Trial registration: Korean Clinical Research Information Service (KCT0005763)