Brisk walking absolutely qualifies as exercise, and should be a regular habit, but it will not serve your body in the same way that running and jogging will. Interval training is the most effective when it comes to toning and burning calories. Perhaps stick to your Saturday walks if you prefer something lighter on the weekends, but then soup up your weekly runs by incorporating sprinting/faster intervals of 20 seconds or so followed by a minute or more of recovering at a slower pace. Training this way will get you into your fat-burning zone faster, where moderate intensity would burn off more carbohydrates than fat. If you want to up the ante on your Saturday walks but not turn them into full-force runs, you could also include intervals of very quick power-walking, followed by intervals at a slower pace.