20 mins of interval training using weights will do more for you then walking for an hour. Your endurance never gets better if you aren't increasing the intensity of your workout. Look at the guys that walk 45mins a day, they plateau extremely fast, like I said before up until a certain point massive amounts of cardio does more for your heart then it does for weightloss. The key is to get your body to burn calories when you are at rest, cardio won't do this.
A good diet, along with 3-4 days a week of interval training will do wonders. You shouldn't need more then 20 mins a day..