This clinical trial evaluates whether prehabilitation with aerobic and resistance exercise improves physical fitness and quality of life outcomes in older patients planning to undergo chimeric antigen receptor (CAR)-T therapy for multiple myeloma that has come back after a period of improvement (relapsed) or that does not respond to treatment (refractory). CAR-T therapy is a type of treatment in which a patient's T cells (a type of immune system cell) are changed in the laboratory so they will attack cancer cells. Large numbers of the CAR-T cells are grown in the laboratory and given to the patient by infusion for treatment of certain cancers. While CAR-T therapy is commonly used to treat multiple myeloma, it can result in toxicities that lead to hospitalization, nerve and muscle impairment, and decreased physical function. Prehabilitation programs use targeted interventions to improve functional status prior to medical or surgical treatments. In this study, patients participate in personalized aerobic and resistance prehabilitation activities in the weeks leading up to their CAR-T infusion. This program may improve physical fitness and quality of life, both prior to and after CAR-T infusion, in older patients with relapsed or refractory multiple myeloma.