If you push to the max in a workout all of a sudden, you risk being injured and then not being able to do anything at all for months and months. Much better to go slowly. Walking is a great idea and probably the best. A recumbent bike is another good way to exercise safely if you have access to it. You also need resistance training. You need a mix of cardio, resistance (strength) and flexibility. I hope you won't end up diabetic and with high cholesterol.