Lots of folks here use different diet/exercise programs to get and stay healthy.
My best tip is to combine exercise with eating right.
Since you'll be doing a lot of walking at the parks, adding in a walk once a day starting now will help you burn calories and prepare your body for all of the walking at WDW. You could also add movement into your daily routine by doing things like parking further from stores so you have to walk further and taking stairs instead of an elevator.
Also, identify the eating issues that have helped to put on the extra weight and try to avoid them. If it's fast food lunch, then start packing a healthy brown bag lunch. If it's hitting the vending machines, then pack a piece of fruit for a snack and don't carry change or small bills. If it's portions, then make a conscious decision to cut your portions to a more moderate amount. Try to concentrate on eating foods that are nutricious -- fruits, veggies, whole grains, low-fat meats and dairy. Avoid foods that are merely fillers -- soda, candy, chips, french fries, etc.
Cutting out/down on soda can trim calories and salt. Even replacing diet soda with water will help you drop water weight from the salt. Drink lots of water to stay hydrated. Don't overdo anything -- take it easy and you'll be able to stick with it easier.
Good Luck!