The key to running the marathon is the long run. You don't have to be fancy about how you do it. You just need to do it.
Jeff Galloway's book, "Running: A Year Found Plan", is a great read. He shares this insight into the purpose of the long run:
"The long run builds endurance by gradually extending slow long runs, you train muscle cells to expand their capacity to utilize oxygen efficiently, sustain energy production , and in general, increase capacity to go farther.... Even when running very slowly, with liberal walk breaks, you build endurance by gradually increasing the distance of a regularly schedule long run. Start with the length of your current long one, and increase by 0.5 to 1 mile per week...."
Personally, I've substituted time as the measuring stick over against a defined mile marker. I increase my long runs by 10 minutes every week rather then a mile. The reason I made the shift is simple, because of weather. I've run a lot of my marathons in the Fall, which means Summer tends to be the time I'm building my distance. Also because I've spent a lot of my summers in nice warm places with temperatures near 100 degrees by 9 am...I felt a need to shift to time. My reasoning was some days the heat and humidity drained me faster and if I was focused on a distance, I would feel bad when I had to abandon the distance just because I was trying to keep a certain pace for the distance. I read an article that argued that time allowed you to focus on going for a period of time with a known end point after which you were finished. If you covered a set difference good, but if you didn't that was still good. It was about moving your body for that time and building on it. I made the shift last summer while in Kuwait and it made a huge difference. I never struggled to finish the run like I had when it was a set distance.
So today as my daughter and I ran, we used Galloway's cycle of run/walk and easily completed the time of two hours. We'll go further next week just because we'll add 10 minutes...and that will eventually get us to the finish line on the 31st of October.