I suggest you find a 5k run/walk in your area that's held about a 4-6 weeks before your trip. Have everyone prep for that.
If they can walk the 5k (3 miles) without being exhausted then they can handle the extra needed at Disney. They'll be tired and footsore eventually, but not likely to point of not having fun. If the 5k is too much, they get a wake up call and still have several weeks to get it in gear.
Also encourage them to spend more time on their feet. Standing still is harder on the feet than walking.
Having a test goal can be fun and gives the newbies a goal to reach for that is attainable and not discouraging. If someone told me I'd be walking 15 miles on vacation (that's more than a half marathon!) I'd think twice about going.
