Seems like Idea knows what they're talking about in their new commercial. Walking is an easy way to burn calories in order to lose weight and decrease body fat.
The number of calories you burn depends on how fast you walk as well as how much you weigh. Apparently, research suggests that a person needs to burn 2000-3000 calories through physical activity every week to control body weight - which works out to be about 60-90 minutes of walking per day.
The minimum duration for which a person must walk in order to lose weight is 30 minutes. Shorter bouts of walking can improve one's overall health but is unlikely to lead to weight/fat reduction.
The good part though is that the 60-90 minutes of exercise can be spread out into small bouts of 15-20 minutes at a time. Walking short distances instead of driving / taking a cab, taking the stairs instead of the elevator, parking your car at a farther spot in the parking lot etc. all help.
Walking can be complimented by making small reductions in one's calorie intake such as skipping desserts after meals, using skim instead of whole milk, replacing sugar with an artificial sweetener etc.
However, if you feel that you cannot start with 60-90 minute walks, you can start with 15-20 minute sessions and add a few minutes every week. The important thing though is that once you manage to lose weight through any form of exercise, including walking, you must continue with it for the rest of your life in order to keep the pounds from piling back on!