I started making some healthier lifestyle choices about a year ago, like completely eliminating sugary drinks from my diet, not eating fried foods, etc. I thought it would be hard to maintain these choices while in Disney, but it is surprisingly easy. Every menu at Disney offers a healthier option, so I usually just take that. For example, order a salad at lunch instead of a burger or chicken fingers (I don't even remember the last time I had a Disney burger or chicken fingers...), order apple slices instead of fries, or just go for the kid's meal. I think that what comes with a kid's meal is more like a normal adult portion anyway.
So, I would not go into a Disney vacation looking to deprive yourself of anything - the temptation will get the better of you at some point, and you will most likely end up eating more than you originally would have. Rather, I would just try and swap out one thing at every meal with a healthier option and stick to mainly water. That, coupled with all of the walking you're doing, will make you feel better about yourself when you get home without feeling like you missed out at all!
