The only way to lose weight efficiently (safely, and making sure you won't just gain it back right away) is to concentrate on 3 factors: 1) The right nutrition, 2) moderate aerobic exercise (walking is good), 3) and a concern for muscle.
You're on the right track with the walking. That will aid in burning calories and will also prepare you for all of your walking at WDW.
For your diet, make sure to avoid fried foods, cream, butter, etc. Foods like that are literally pure fat and will basically cancel out all the walking you are doing. Also avoid simple carbohydrates as much as possible (white rice, sugar, white bread, etc.) If you eat whole wheat breads, pastas, and brown rice, those foods will actually aid in increasing your metabolism because they are complex carbs which are difficult for your body to break down. Also make sure to eat plenty of fibrous vegetables and lean proteins (white meat chicken, lean beef, fish, etc). As long as you eat the right foods in balance, your body will actually burn calories when it breaks down your meals, and in turn speed up your metabolism. It's more important to eat the right foods than it is to count calories.
Your weight loss will speed up if you do some light weight training or bodyweight exercises (like sit ups, push ups, or leg lifts) before you go out walking. You'd be surprised at how quickly you will get stronger and build some lean muscle. Lean muscle is essential, because all of the fat burning in the body takes place in the muscles. The more muscle a person has, the higher their fat burning capacity will be. If you do some moderate weight training right before you walk, you will increase your capacity of burning fat as you walk.
Hope that makes sense, lol. Sorry for writing you a novel. I think I basically regurgitated a few chapters of my personal training manual. Good luck!
