Great advice so far - drink plenty of water, walk, and don't be afraid to make special requets.
The fast food at WDW is, well, pretty fast food-y. However, at sit down restaurants you should be able to find items that fit within WW points, or make special requests to accomodate your needs.
It's been a while since I've visited a WDW park, even though I grew up in Central Florida
- I recall that the Japanese Pavillion at Epcot had a nice fast food restaurant with Noodle Bowls, sushi, etc. The hotels on WDW property usually have incredible restaurants, and as noted above, you should be able to make requests as to cooking methods and substitutions. Many of the restaurants feature great seafood dishes, which are usually on the healthy side.
I would also recommend packing some healthy breakfast items and snacks. If you're staying near a grocery store, stock up on bottled water, fruit, yogurt, etc.
Hope you have a great time!
-- Tally