Double, exactly. There is way too much focus on the long run these days, to the detriment of some actually more important factors in marathon training.
Garboni, I understand where you come from but I think the 20 mile long run is almost completely a psychological thing. Not that the mental aspect of training doesn't matter but how much of a difference is there between, for example, a 60 mile per week training plan with an 18 mile long run or a 50 mile per week training plan with a 20 mile long run? I'll take the 60 mile per week plan every time.
One thing to keep in mind, as referenced in this Joe Henderson archive, is the collapse point theory. I'm becoming a bigger fan of this concept all the time. I'm really convinced that running about 60 miles per week (roughly averaging 1/3 of the race distance per day) for an extended period of time would do more for most runners than doing a couple 20 mile long runs.