As if you needed a reason to walk not drive around Toronto, the Wall Street Journal writes that walking is a good way to increase your mental fitness:

The key to keeping intellectually sharp as we age may not be mental gymnastics, as commonly recommended, but real gymnastics.
According to a new study, the brain's long, slow decline may not be inevitable. For the first time, scientists have found something that not only halts the brain shrinkage that starts in a person's 40s, especially in regions responsible for memory and higher cognition, but actually reverses it: aerobic exercise. As little as three hours a week of brisk walking -- no Stairmaster required -- apparently increases blood flow to the brain and triggers biochemical changes that increase production of new brain neurons.