It's important to understand that slow long run can have some effect at the beginning only, not for long time. To overtake personal limits means to have mentality of an explorer, going sometimes in a dark area you don't know. This is the same in every field of the life : when you want to improve in any type of knowledge, you must explore something you don't know, otherwise there is no reason for any improvement. This is about education and culture, this is about physiological changes, coming as ANSWERS to the PROPOSAL, able to STIMULATE your body and your mind. Every kind of stimula is only in 2 direction : EXTENSION, or INTENSITY. So, everytime you do something new, you go to stimulate your body. This is the case of EASY LONG RUN, too : if never you ran longer than 1 hr, the first time you move to 1 hr 30, 1 hr 45, 2 hrs (also slow), you are doing something NEW, and this fact can stimulate your body. Bud, because running long at moderate pace is very easy, after very short time every stimulating action is over, so THE CORRECT WAY FOR CONTINUING TO IMPROVE IN THE DIRECTION OF SPECIFIC MARATHON ENDURANCE IS TO MOVE, YEAR AFTER YEAR, TO MORE FAST SPEED, MAINTAINING THE SAME DURATION.
This is because we use, far from the specific race, at the beginning of the season and when you are still young, training of DURATION, where the most important goal is running very long, but not fast. This because we have to build adaptation (mental and in the body, as tendons, ligaments, technical economy), BEFORE STARTING THE REAL SPECIFIC TRAINING FOR MARATHON.
Only later we need to qualify long run, and DURATION becomes DISTANCE. In this case, we decide the speed (normally close the speed of the marathon, in any case not slower than 95% of the marathon pace), and we control the ability in extending that speed in different ways : even pace, alternated pace or progressive pace (in any case with a final average between 95% and 100% of the Marathon pace). DISTANCE cannot be shorter than 2/3 of the total mileage (28 km) and longer than 38 km. Some time, we can run longer than 45 km (Gelindo Bordin used running 3 hrs two times before his top marathons, for about 52 km), of course at a pace about 90% / 93% of Marathon pace.
An athlete becomes Marathon runner when is able to change his metabolism, using a higher percentage of Fatty Acids (consequentely lower percentage of Glycogen) for the same speed, that is the Marathon speed. As the tank of Fatty Acids is very much more capable than the tank of Glycogen, when an athlete reach that goal, automatically can last longer at the same speed.