How long has it been since you have exercised? How old are you? What physical activities have you participated in the past? What are your eating habits like?
I am all for jumping in with both feet, but you have to be careful not to "over-do-it" in the beginning of your training. It starts with diet. You have to reduce your calories and increase your physical activity.
Consistency will help you toward your goal. You are talking about a 20% reduction in body weight. That is pretty significant, and it won't happen overnight.