Analysis of 20 longitudinal studies reveals that sustained habitual physical activity preserves hippocampal and prefrontal gray matter volume while attenuating white matter microstructural decline. Executive function and processing speed showed the strongest cognitive associations, with evidence of dose-response effects accumulating across adulthood. Notably, benefits appeared amplified in APOE-ε4 carriers, the genetic variant conferring higher Alzheimer's risk. This finding reinforces emerging evidence that exercise may be particularly protective for those genetically predisposed to dementia. The bidirectional nature of the relationship—where preserved brain structure also predicted subsequent physical activity—suggests the brain-exercise connection operates as a positive feedback loop rather than simple cause-and-effect. While the observational design prevents definitive causal claims, the consistency across studies and dose-response patterns strengthen the biological plausibility of exercise as a neuroprotective intervention. For adults concerned about cognitive aging, this meta-analysis provides compelling evidence that maintaining regular physical activity throughout life may offer measurable protection against brain atrophy and cognitive decline, particularly in executive domains critical for daily functioning.