I did WDW as my first marathon and I loved it.
Having said that, the half marathon is a commitment of like "we're going steady" and the marathon is like "We're getting married." Depending on how I train, it sucks up my entire life and especially time on the weekends (which for WDW is prime holiday season). I get cranky and annoyed, spending what feels like all my free time running and like I can't go out and do other more fun things. So, you know, it's rewarding but once the mileage starts to climb it really sucks up your entire life.
Plus, the weather can be a total crap shoot at Disney and there's nothing worse than training in the chilly dry 30s/40s and coming to Florida to run in the humid 60s/70s/80s.