Well, in order to use brisk walking to burn a good amount of calories you'd have to walk for 70 minutes or more. 30 to 40 minutes a day will certainly help you keep active and in shape, but 70 would be better for fat loss. Walking is a great exercise, it may be a little weak for some, but it's a great general exercise. Especially if you can't force your knees too much.
To be frank I am not keen to take walking as a form of exercise as it burns fat very slow. If you are a fatty person you should follow other more rigorous workouts to burn those extra fat. If your fat intake is higher than what you burn through workouts you will become obese over a period of time. But if you are physically fit and BMI is well within the recommended range brisk walking along with other exercises can help you maintain your weight. I walk whenever I have a chance to do so. If it is not a long distance to cover I prefer go on foot than using a vehicle.
