Okay, a lot of variables at play here obviously:
1. The runner's background (experience marathoning, average weekly mileage per year, number of years running, FT/ST muscle fiber ratio, etc.)
2. Goal marathon pace versus actual marathon pace fitness (is the goal pace reasonable based on half marathon race performances and training base).
For someone running under 2:30 in the marathon a run of even 13 miles at current MP pace is going to be very taxing...near lactate threshold pace/effort and not to mention the skeletal/muscular fatigue is relatively high.
For someone running a 3 -4 hour marathon a 13 to 20 mile long run is quite doable although running shorter and slightly faster workouts may have more bang for the time spent. The 20%-25% of weekly mileage for the length of the long run is a constant that we can and should use though.
Taking Daniels and Pfitzinger's guidelines into account (and heck Hansons marathon plan for mortals) the PACES for easy aerobic days and long run workouts have a rather large variance. Essentially every training variable here with pace/effort is exponential. The percentages faster than marathon pace should vary accordingly (shall not be constant) and also be considered with the length of the run and the weekly mileage of the athlete.
SO...just run by feel!
In a nutshell I would agree that most people run their easy days too hard. Also, for the beginner marathoner just getting their mileage up and staying injury free (along with some shorter and faster lactate threshold workouts) will bring the marathon time down rather than trying to hit race pace or faster on every run.