I walked 0.2 miles yesterday and was overly fatigued, what could be the problem. i'm 40 yrs old and pretty active?

There are many possible causes of your fatigue while walking including dehydration, poor nutrition, lack of adequate sleep, iron deficiency, a thyroid condition, medications, cancer, an infection & a cardiac condition. I suggest a general exam with a primary care physician to check for any possible medical conditions. I wish you well with your health!