The first time that I heard about this system (intensity before quantity, for marathon runners) was in 1972, from Belgian Gaston Roelants, one of the greatest runners of the period. Roelants was record holder of steeple, then moved to marathon. At that time, he was already old (35 years), and, when he explained me his program, I also asked him "why speed before endurance ?". His answer was : "For me, running long and slow is very easy, after many years of activity. What I lose in short time is the capacity of running fast. So, for me it's a mistake running many km slowly before developing speed. When I'm able to run 10 times 1000m in 2'48" recovering 1 min, for me is very easy, in short time, running 20 km at 2'58" and full marathon at 3'10"".
Going on with our experience, we applied this system with all italian runners, from Pizzolato to Poli, from Bordin to Bettiol, from Baldini to Caimmi to Giacomo Leone now.
Currently, long and slow run doesn't exist in a modern training program for marathon. Really, about 40-50% of the global volume is done with slow run (under 80% of Marathon Pace : for example, if you run a Marathon at 3'10" per km, 19" every 100m, 20% is 38", so running at 3'50" isn't a work, but regeneration run), but TRAINING FOR BUILDING YOUR PERFORMANCE BEGINS WHEN YOU RUN OVER 90% OF M.P. (from 3'30" going faster). Specific workouts for marathon are WORKOUTS LASTING MORE THAN 1:30:00 at a pace of 90% or better, and in any case never shorter than 22/24 km.
In our methodology (the same for Spanish runners), at first we try to lift Anaerobic Threshold (about 60-45 days before the marathon, difference between AT and AnT may be about 10%), then, maintaining the same intensity, we try to develop specific endurance.
In the modern concepts for middle distance, WE TRY TO DEVELOP THE POWER OF OUR ENGIN (that is Anaerobic Endurance) in the first 2 months of preparation, then TO DEVELOP SPECIFIC POWER-ENDURANCE GOING TO THE EXTENSION.
This is a system not only for marathon, but for all long distance. Of course, is yet possible to use the other system, but the capacity of intensity cannot stay long time without beeing stimulated, and is a mistake to permit that your quality can go down too much.