Hi Michael!
Thank you for bringing your question to the Disney Parks Moms Panel!
The official runDisney training expert is
Jeff Galloway. As a former Olympic athlete, lifetime runner, running coach and running book author, he really knows his stuff! I have asked him advice personally before and the one thing that stuck with me is that training (for most people) is really all about getting the time on your feet. You don't have to be fast, and you can use run/walk intervals, but you have to get your feet and legs used to running for long periods of time.
The additional mileage is there to really get yourself prepared for the distance. Some training programs have you go beyond the distance, while others stop you short. It's really a personal preference. I have trained with both. One thing I will say about races that I trained for by running longer than the race distance is that my confidence level was much higher for those. Running success can depend a lot on mental attitude. Knowing that I have recently run farther than race distance really helps me be less nervous on race day and in turn, helps me to enjoy the whole experience more.
I hope this helps! Please let me know if you have any other questions.