Walking is effective for the elderly and the obese.
For exercise to really be effective you must increase your heart rate for at least 30 minutes.
Good workouts can be productive and addictive. The more you exercise and see results, the more you want to workout. The problem is not allowing your body the proper rest can be counter productive. Your body needs rest days and the proper nutrition to be able to rebuild it-self and get ready for the next challenge. Muscle damage, heart problems, high cortisol levels, free radical cell damage, and a host of other health problems will occur if you don't properly space out your workouts. Be smart and listen to your body, REST!