[Slick]

I have to disagree that walking is "the better choice". Yes, bikes are more efficient for getting from point A to point B, but they are comparable when you excercise for the same length of time. Your mathematical example is based on distance when it should be based on time. I suspect that a calculation based on time would show that you expend more energy per second on a bike. That's not to say that biking is "the beter choice" either. Either can be appropriate to an individual's needs. There are tons of articles on the subject.

You'll get your heart rate up higher on a bike and you will also exercise more muscles in your body. Cyclists also build up more coordination skills as a side benefit. You probably stand a higher chance to get injured, or more severely injured on a bike also.

Walking is better for low impact, low heart rate excercise. Older people, people with certain problems, etc. would find walking better. Doctors often prescribe walking because of its high benefit/risk.

They are simply different and it really depends on what you are looking for, your overall health, and what you enjoy enough to do regularly. (Cyclists will normally end up spending more money on bike stuff as time goes on; that may be a concern to people too.)