Another reason you should go for a walk today: Putting in six to nine miles a week may help you remember that you left your keys in the freezer. This from a University of Pittsburgh study published in the journal Neurology that followed 300 seniors whose average age was 78 at the beginning of the study in 1989. Over time — about 13 years — one-third had developed “mild cognitive impairment or dementia.” But testing showed that more active walkers in the group had more gray matter and thus better cognitive skills. Thus, they were able to remember that after getting home from the grocery their hands were full as they pushed their way through the front door, climbed the stairs, entered the kitchen, shimmied the freezer door open with an elbow and put the ice cream in the freezer — along with the keys they were still clasping in their right hand.