Exercise and Concentration

As I suffer through my morning exercise routine of stationary bicycling, I think of a lot of things. My mind races. And this morning, my mind pondered the following question: Why is it that when I exercise I can have intense powers of mental focus (either on a work task that needs to be addressed, rehashing an event from the past, formulating responses in my mind to blog postings I disagree with, concentrating on the rhythms and instrumental arrangements of the music I'm exercising to, etc.); and yet still be unable to focus my mind on what I would consider to be similar kinds of mental activity such as reading.

As much as I would love the distraction of being able to read a gripping novel, magazine, or some other engaging print media when I am exercising, I just can't do it. That kind of mental activity I seem to be incapable of sustaining. In fact, I would say that attempting to read and exercise at the same time induces a kind of heightened physical discomfort. And yet I see people at the gym all the time who pedal the stationary bike or run the treadmill and read at the same time. Makes me wonder if there are different parts of the brain that do different kinds of focusing and stimuli processing such that physical activity can interrupt the functioning of one kind of mental focusing activity but not another. In other words, why can I rehash a the complexity of a blog discussion thread in my head (and even add new ideas or thoughts to the mix) for extended periods of time when exercising, and yet can't sustain the mental focus to read through even a paragraph of printed text while doing the exact same exercise?