Aerobic exercise induces glucose repartitioning away from breast cancer and melanoma tumors in rodents. Voluntary wheel running slows tumor growth in mice. Exercise shifts glucose uptake and oxidation to skeletal and cardiac muscle. Glucose metabolism reduces in tumors during exercise. Fitness predicts tumor metabolism along a continuum. Prehabilitation in obese mice enhances muscle glucose uptake and reduces tumor glucose use. Muscle glucose oxidation increases while tumor glucose oxidation decreases with exercise. Slower tumor progression correlates with reduced tumor glucose metabolism. Exercise downregulates mTOR signaling in tumors. Exercise creates metabolic competition that constrains tumor energetics. Higher exercise capacity improves cancer prognosis at all stages.