The above article in MedPage Today offers anecdotal evidence of what many clinical trials are finding about the benefits of exercise, even just 45 minutes per day. Two men, one diagnosed with MCI at age 64 and one at age 72, continue with minimal change in cognition through their current age of 80. One plays tennis 4-6 times a week for two hours at a time; the other uses an elliptical, treadmill, and/or Peloton daily for 3-4 hours and plays tennis or golf when possible.
I doubt we all need to buy Peloton machines or become tennis pros. It seems to the level of sustained activity over a long period of time, not one specific activity, that MAY be important for people with MCI. Presumably diet, sleep, social supports, etc. also were in good shape for these men, but were not where they put their efforts.
Here's what the authors of Vigorous, regular physical exercise may slow disease progression in Alzheimer's disease think explains their stable levels after receiving a diagnosis of MCI:
Both patients regularly exercised vigorously for hours a day and increased their participation after they either retired or reduced their work hours..."The two patients in this report had Alzheimer's brain pathology based on established biomarkers and clinical features but showed little cognitive decline for 16 years and 8 years, respectively," [said co-author Davangere Devanand, MD, of Columbia University Irving Medical Center in New York City]. "Vigorous, regular exercise was the most likely explanation for their lack of deterioration, because they had multiple medical problems and did not focus on diet or cognitively-stimulating activities beyond what they did earlier in life."