I have run the Half in 2012, Full in 2013, and the Dopey the last two years. Our group has stayed at POR each year and have really enjoyed it; transportation works well, the dining options are nice, and taking the boat to Downtown Disney/Disney Springs is a nice alternative to bus transportation. Every year we consider going somewhere else for a new experience, but we end up going back to POR. I think we would have an equally great time at any other resort we could choose but, as you have seen from other posters, poor experiences seem to be few and far between at any of the Disney properties. As for what/where to eat, that's personal preference. Our group has chosen to eat on the Disney Properties (Resorts, Parks, and Downtown Disney) so I am sure there are better price conscious options, but we really haven't had any bad dining experiences. Like everything else at Disney, you are paying a bit of a premium; but they tend to do things right.
After the half and the full, my first race at both distances, I would not have wanted to board a plane the next morning because of stiffness/soreness. After the Dopey the last two years my legs have felt a lot better. This past January my flight home was at 10:15 Monday morning and I felt fine (other than adding another early wake-up to my "vacation"). So I would say that depending on level of fitness and comfort with the marathon distance physically flying home Monday is not a problem. However, each of the other years I have stayed at least one extra day to relax, go to a park, golf, etc. and that has been much more enjoyable. So it's really all about what you want to do and what your schedules will allow.
Our group has arrived Wednesday between Noon and 3:00 PM the last couple years. It has given us plenty of time to get to the expo, get unpacked/settled, and have a nice relaxing dinner before the first of our four early bed times. Getting enough sleep and just getting out of bed at ~3 AM each day is the most difficult part of the trip for me.