I think you should walk very fast for at least a half an hour. If you were to speed walk for one hour everday, then your heart rate would eventually go down and level out. Also you would lose tons of pounds!
Anna:)
If you want to burn lots of calories then duration is the key. You don't want to get your heart rate too high because if you do fatigue will set in. Take 220 and subtract your age. That is your maximum heart rate. Then I would say multiply it by 70%. I would work to stay at that heart rate for 45 minutes to one hour. You should easily be able to talk. If you do that 5 days a week that should be sufficient.