In my opinion, if you are planning on running 35-40 mpw then doubles would probably not be as beneficial as single longer runs. I have found I am less injury prone when I build mileage more than 50 mpw in doubles, but I have read many articles/books suggesting this is the case when you hit 70 mpw. I think the discrepancy is in counting miles and not "time" (for slower runners like me there is at least a 2 minute/mile difference in "easy pace" - which adds up over the course of a week) -- so, it can be interpreted as: doubles could make it easier (on your body) to increase mileage if you already run about 8-10 hours/week in singles. I am sure there are other benefits (reference to Tinman's article about running efficiency), but my main concern is keeping injury free while trying to increase mileage.
Personally, I think a few months of slowly, gently increasing your mileage (at a slower speed) will allow you to run more miles without injury - meaning I don't think you will be limited to 35-40 mpw, which will be more conducive to a faster marathon. But, you will have to be patient (and not look at the mileage board as often
).
Paul's schedule is interesting -- Brad Hudson suggests this (doubles on easy days and workout days as a single) - but Jack Daniels (and by extension my coach) and others prefer to have doubles scheduled on the same days as harder workouts. And then there are many other elites, who run doubles (or even triples in the case of some of the Kenyans and Ethiopians) every day except for their long run days.