Ah, the long run. No endurance runner can do without it. The long run is so essential to your success in the marathon that you've got to plan it into your training schedule very carefully. Ignore it, and you will likely encounter a much more difficult marathon come race day. Pay attention to it, and your race should go much better.
My next long run is this Saturday, Lord willing. It's a half marathon trail run at the Pocahontas State Park near Richmond. Today I checked the temperature, humidity, chance of precipitation, and wind speed, and everything looks great. My plan is to run slowly and conservatively, probably with a mixture of running and walking even though 13.1 miles is a distance I can usually run without stopping.
Half marathons are special. They shine a spotlight on your level of fitness and offer a sort of "dress rehearsal" for the marathon. Each hour, you lift your feet up and down nearly 5,000 times. The long run forces you to pace yourself so that you don't hit the wall before the end of your marathon. Any half marathon you finish feeling strong is a good one, no matter how fast or slow you run. As this is a training run, I have no time goals. I want to pay attention to the small stuff: form, cadence, nutrition, hydration, and recovery. The last 3 miles won't come easily, but you get through them because your training has made you strong enough to endure.
For developing the aerobic engine, nothing beats the long run. Running a half marathon is a beautiful act of the will. You must let go of the urge to run too hard and too fast and accept that your strength is gradually building up to the point where you will be able to complete your target marathon.
Truly, the long run is a beautiful thing.