I disagree. Walking is good, but if you're really overweight then lose it by diet first, before really focusing on excercise. It will be so much easier when you're not carrying all that weight around. I lost 30 pounds this summer on a keto diet with almost no excercise. I now have a ton more energy, and I'm starting to get into shape.

Gary Taubes points out in Why We Get Fat that exercise builds muscle, but doesn't lose fat. He argues you do that best by cutting carbs.