Here's my opinion on walking to cut fat. Basically, during low intensity activities, fat is one of the preferred sources of energy (Notice I said one of; glycolysis is still going to be of significance, but probably not fermentation). Okay, so during your walk, you are using a significant amount of fat to power this activity. Great.
You now have to look at the flip side. Assuming you are performing higher intensity cardiovascular activity, your body will draw energy primarily from fermentation and glycolysis, meanwhile utilizing aerobic energy pathways to a lesser degree. Okay, so more glucose is broken down for energy as opposed to fat during the activity.
Now let's look at the effects that occur after the workout ends. A sustained elevated metabolism seems to depend on two factors: intensity and duration. Intensity appears to be the primary determinant of how much and how long your metabolism stays elevated after a bout of exercise. Therefore, a long walk probably has an almost negligible effect on your basal metabolic rate after the workout has ended, at least relatively speaking. However, higher intensity exercise should have a very significant impact.
What does this all mean? The point I am trying to drive home is that although you may be utilizing a significant amount of fat for energy during the walking itself, you are also utilizing a good amount of fat for energy during your everyday activities. High intensity exercise allows you to perform these lipophilic, everyday activities (Which probably includes a significant amount of walk) with a steadily raised metabolic rate.
Beyond all this, I think high intensity cardiovascular activity has a place in anyone's routine. It is associated with promoting a greater usage of fatty acids in cellular respiration. By the same token, it promotes greater storage of glucose in skeletal muscle; in the form of glycogen of course.
Perhaps the two could be combined in some optimal manner? Perhaps walking is still superior despite these facts? I can't say for sure. However, it is my theory, based on this evidence, that high intensity cardio is superior in terms of promoting a body composition of lower body fat.