I think it is very easy to eat healthy at WDW if you make that a priority. Here are a couple of tips to help you along:
If you are eating at a buffet remember to practice portion control. Ideally you would not have the worst offenders on the line and stick with the healthier options. If you want to try some of those high calorie offenders take small portions. I like to use a salad plate instead of a dinner plate because it will make smaller portions appear larger
Disney is very good about substitutions and alterations. A meal that comes with a heavy sauce can be made much healthier with that sauce omitted. The same goes for side dishes. Ask for vegetables instead of fries at the TS restaurants.
Look out for desserts. If there is a fresh fruit option of a sorbet take that over the cheesecake. If there are two of you and you really want the cheesecake get one and split it.
There are some restaurants that have more healthy options than others. By top picks are Boma, Tusker House, Restaurant Marrakesh, The Wave, Kona, the sushi restaurant in the Poly, and the Garden Grill.
If you are going to be doing more walking at WDW then you would during a normal day you can increase your caloric intake in proportion.
Here is a chart showing about how many calories you burn per mile based on pace and weight. Lets say you weight 140lbs and walk at 2mph. That means you will burn 80 calories per hour. If you normally walk 2 miles per day but at Disney walk 5, that mean you can eat an extra 240 calories and keep your calories in vs. calories out the same. A mistake a lot of people make is thinking they are walking twice as much so they can eat twice as much. That will just result in a calorie surplus. A pedometer can give you an accurate pace and total distance.
The first and most important thing you are already doing, and that is being conscious of the pending pitfalls and planning accordingly.