How exercise could make you smarter

According to an article in the NYT, after running experiments with mice for months studies have concluded that physical exercise could give humans brainpower that thinking does not. Why?

Source: letsmakeahealthiertomorrow.tumblr.com via j jump jennifer on Pinterest

The brain, like all muscles and organs, is a tissue, and its function declines with underuse and age. Beginning in our late 20s, most of us will lose about 1 percent annually of the volume of the hippocampus, a key portion of the brain related to memory and certain types of learning.
Exercise though seems to slow or reverse the brain’s physical decay, much as it does with muscles. [...]

Even more heartening, scientists found that exercise jump-starts neurogenesis. Mice and rats that ran for a few weeks generally had about twice as many new neurons in their hippocampi as sedentary animals. Their brains, like other muscles, were bulking up.

But here is where it gets interesting: the brain cells that are a product of exercise are nimbler than those created by cognitive tasks. In other words, it is true that if you learn new things (say a new language), you reverse the aging process of your brain, but those neurons are fired only when you practice that language. Instead, the neurons you get from exercise can multitask (at least for mice)!

I wonder if this applies only to aerobic exercise; the article mentions that which type of exercise is the one is  "another intriguing issue". It is prove, though, that it does not have to be exhausting; research shoes that going out for long walks regularly has worked for the elderly (I can include my grandma as a personal observation here).

And I was thinking of skipping the gym today... hmm...