If you are going to walk, instead of any other more strenous type of cardio training (running, cross country skiing, swimming etc..), then use a treadmill for best results. This way you can put up the incline and not even notice it(just put it at 3 or 4%).
The treadmills at my gym go up to 15% incline and I can run at that, albeit not very fast but long if I choose, it all is individual. Your body will burn more calories if it thinks it is walking up a slight or hilly or both type of hills. You can , and should, also vary the speeds and length of these adjustments (called a fart lick in running) another advantage of a treadmill.
I would guess you can burn up to 40% more calories with a proper regiment of walking on the treadmill then walking around your block at the same speed! I would also throw in some running, a minute here and there if that's all you can do. If you can run more do it here and there, your muscle will not waste away! Cross training is the best as it has to many benefits to get into. Also do your cardio in the morning on a empty stomach or after your weight session, or both. I think we all know that.