In the movie Back to the Future part III, Doc Brown is sitting in a saloon in the Old West divulging some details about the future to the saloon's patrons: "In the future, we don't need horses: We have motorized carriages called automobiles."
After some bemused chuckling, a man with an asthmatic wheeze in his voice asks, "If everybody's got one of these auto-whatsits, does anybody ever walk or run anymore?"
"Of course we run," replied the doctor, "but for recreation, for fun."
"Run for fun?" the same man asked incredulously "What the h#%% kind of fun is that?"
This quote reminds me how strange it is that the same kind of primitive locomotion that humans used for centuries to get around is now mainly reserved for exercise and for recreation. One reason I love bicycle commuting is that, not only does it get me where I'm going relatively efficiently, but it is practically my only source of aerobic/cardiorespiratory conditioning at the same time. The 6-8 hours I log in my bike during a work week (and that's not even counting trips to the grocery or to friends' houses) is more than enough to keep me in good aerobic shape. I can count on one hand the number of "loops" that I've ridden in the past year (by "loops" I mean rides with no destination done purely for training, in which I start and end at home with no intermediate destination). Even when I ride on days off, I will still pick a destination (such as a restaurant) to which I will ride, spend a few hours there, and then ride home. As for running, I run when I have to in order to pass the Army Physical Fitness Test, which my cycling keeps me in more than good enough shape to excel at. While I'm an okay runner, I don't enjoy running very much anymore because of how inefficient it is; adding 25 pounds of mechanical advantage gets me to my destination at twice the speed with less effort expended. (The only down side is that cycling doesn't build much bone density unless one stands up to pedal quite a bit).
So this is sort of an informal poll of bikeforums commuters: Is your commute your main source of aerobic fitness? Do you still ride or run on your off-time to keep yourself in shape?