A SMART Trial of Adaptive Exercises to Optimize Aerobic-Fitness Responses

The goal of this clinical trial is to test 6 months of aerobic exercise in older adults who are 65 years or older and have mild cognitive impairment (MCI) or probable/possible mild Alzheimer\'s Disease. The main questions it aims to answer are: * test the effects of aerobic exercise on aerobic fitness, white matter hyperintensity (WMH) volume, and patient-centered outcomes; * identify the best exercise to improve aerobic fitness and reduce non-responses over 6 months; and * examines the mechanisms of aerobic exercise\'s action on memory in older adults with early AD. Participants will receive 6 months of supervised exercise, undergo cognitive data collection and exercise testing 5 times over a year span, have an MRI brain scan 3 times over a one-year span, and have monthly follow-up discussions on health and wellness.