There is always a physiological benefit to running long, but I think the overwhelming benefit is mental.
The long run is essential to marathon training. In the shorter distances, you are regularly covering the distances you are racing. In the marathon, you aren't. Consequently, if you start a marathon at 10-K pace, you are headed for a disaster.
The long run helps in understanding pacing. Though you aren't running the long runs at marathon pace, you begin to appreciate the distance and what it requires. This is why I believe your long run should be three hours or more and not two. Others may disagree. I just think you should run to a point where you are pretty much wasted.
If you're not running the marathon, I don't see the point of the long run. But that's because I see it more as a mental thing than a physical thing.
I don't know how to describe it except to say this. If you don't do long runs and then run the marathon, don't be suprised if at some point you say to yourself, "Damn, I f***ed up. This shit is not what I expected. I drank too much at that last aid station. I feel a shit coming on. I have a blister with these flats that I've worn for every other race with no problem. I'm hitting the wall, and I don't know what the f*** to do. Jesus H. Friggin' Christ."
I liken doing the long run to f***ing a blow up doll before plowing into the real thing. You can get your techniques down and determine if you have the stamina to make it. Then, try your skills with some flesh and blood skanks (tune up races) to help even more. Then, hit the real deal.
Heh heh. . .I wonder if this will make the front page.
Out.