I'd recommend Lynne Robinson's 'BodyControl The Pilates Way'.
I started exercising to this DVD in January (on the recommendation of my physio) and the results have been excellent.
I have joint problems, so high impact stuff is out, but Pilates is fantastic for toning everything up and my stomach has never been flatter - given that I'm 41 and a mum of three, that's nothing short of a miracle lol!
I also second walking, as part of my weight loss programme I fit in three hour long walks every week and it helps to shift the pounds. Walking at a brisk pace will burn fat, where as Pilates will tone you up but won't shift many pounds.
Good luck