Everything I've read has said to do your long runs 45-90 seconds per mile slower than your marathon pace. I didn't do this when training for my first marathon. I pretty much ran every run at the same pace.
I noticed that one month after the marathon I had way more "speed" than I had for the marathon and my legs felt fantastic in comparison to how they felt on race day. Some of the reading I have done in preparation for my next marathon indicates that how I trained probably was the cause of this. The experts also say that the benefits of the long training runs for marathoners don't all really kick in until your run duration has reached 90+ minutes.