Maintaining a 16:00/mile pace isn't too taxing, but it would still add up over the course of 13.1 and 26.2 miles, let alone the two on back-to-back days. And it wouldn't leave much time for bathroom stops (or photo ops). Professional speed walkers might be able to keep a faster pace, but most people's version of speed walking probably isn't going to approach 12:00/mile (5 mph), and attempting to do so could lead to an unnatural gait that could lead to injury over that length of time.
Disney's pace requirements is lenient in the grand scheme of pace requirements out there, but don't mistake that for meaning it's something that can easily be done -- even those planning to walk it should train for the longer distances to help ensure that they can maintain the pace for a good length of time and, perhaps even more importantly, avoid injuring themselves in the process.