Background: While exercise is critical for breast cancer survivors, identifying prescriptions that promote long-term adherence is a challenge. This study evaluated whether survivors could maintain fitness gains during a 6-month self-managed phase following two distinct 12-week supervised programs. Specifically, we compared whether transitioning from one-on-one training to either individualized (Ind) or group-based (Gr) sessions influenced a participant’s ability to sustain improvements in strength, range of motion (ROM), and cardiorespiratory endurance (VO₂peak). Methods: Thirty breast cancer patients from diverse backgrounds completed an initial 12-week supervised individual training program. They were then randomized into either Ind (n = 13) or Gr (n = 17) supervised sessions for a second 12-week phase. Fitness assessments were conducted at baseline, 3 months (post-initial training), and 1 year (after the 6-month self-managed phase). Data were analyzed using generalized estimating equations to evaluate the effects of time and training format. Results: Significant improvements were observed across all fitness categories over time. Muscular strength (bench press, plank, and squats) and VO₂peak increased significantly by 3 months and were successfully maintained at the 1-year follow-up mark. Shoulder ROM also showed significant improvement at 1 year. Notably, the training format (group vs. individual) had no significant impact on these outcomes, with the exception of one ROM metric. Conclusions: Initial supervised exercise leads to significant fitness gains that breast cancer survivors can successfully maintain for at least six months through self-management. These gains are sustained regardless of whether the preceding supervised training was delivered in a group or individual format, suggesting flexibility in clinical exercise prescriptions.