Although walking isn't considered as strenuous as jogging, it can be almost as challenging. Why? As you walk faster (speeds above 3mph) the length between your strides increases which is more inefficient than a more natural walking stride. This requires more energy to move your legs, leading to more upper-body rotation resulting in a greater demand on the body and thus the aerobic system, burning more calories in the process. To put it simply, doing it the wrong way is harder.

Tests have confirmed that oxygen consumption for runners is only slightly lower for competitive walkers than it is for runners. Oxygen consumption is a good yardstick to measure an athlete's aerobic outpu. You may also be surprised to learn some racewalkers can reach speeds of 9mph.

Source: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1859672