Amazingly enough there is not a whole lot of exercise required for this diet to work. According to everything I have read, cardio is preferred over muscle building exercises while on this plan. You want to lose fat not tone it....you can work on your muscles later!
So walking is perfectly acceptable during phase 1 and 2 of the program.....and if you choose you can add more exercise during phase 3 but before that you will not be eating enough calories to work out really hard...take it easy and enjoy the pounds slipping off without much effort. I'm all for not exercising and losing weight....how about you?