I am with Mr. Lindgren on this point.
What I find myself is that the most effective and aerobically-stimulating long runs are the 2;30 to 3 hour long moderate efforts (about 6:15/mile for me, or about 1 minute per mile SLOWER than my marathon pace)
I find that I ALWAYS have a great workout 2 or 3 days after this sort of run, but again you have to keep the pace easy-to-moderate.
When I ran 60:34 for 20km in the the Tokyo Marathon last year, I had run 44km in 2:45 just one week before.
When I ran 48;33 for 10 miles in training last year, I had run about the same workout, 44km, only 3 days earlier.
For the marathon, you have to do only 2 kinds of workouts:
(1) Workouts that get you to the finish line (Long runs and overall training volume) and
(2) Workouts that let you run the pace that you want to run in the marathon. (tempo runs of 5km-25km and intervals)
If you learn anything from me, please learn from my mistakes. If you mix these two together a lot and run balls-to-the-walls 40km's, you will be too tired for too long. Why does Khannouchi run 35km hard runs and not 40km? Because when you get past about 32km in a hard pace, the rate at which you chew up your body is greatly accelerated.