I applaud you for your dedication! While there are certainly temptations abound at Walt Disney World, I also think it may be one of the easiest places to make healthful choices.
First, you'll do a significant amount of walking (bring along a pedometer to actually see how much you walk). If you happen to have taken up jogging, many of the resorts have jogging paths for guests to enjoy as well as fitness centers.
Disney truly has impressed with me with their commitment to providing a variety of healthful food choices. Throughout the parks, you'll find fresh fruit/veggies stands. In fact, at the counter service restaurants, fries are not the standard side. You'll have the opportunity to order items like apple slices. Also, a bottle of water is always an option instead of a glass of soda there as well. By the way, if you are on the Disney Dining Plan dessert is included for lunch & dinner. Don't hesitate to ask for fruit or a healthful alternative to what is on the menu.
I think I'd personally steer clear of the buffets since, at least for me, those are easy to overindulge. One restaurant that you may want to visit is
The Wave, which specializes in healthful food. You won't find any fried food there!
Disney Mom Carrie has also offered her
suggestions on staying healthy while at WDW. Similarly, if you visit www.disneyfoodblog.com and do a search for "healthy choices," you'll find many more articles on eating well.
Have a great trip! Please let us know if we can help again.