Three things I have discovered while training for the San Francisco Marathon:
The importance of breaking a big problem into many small, solvable chunks.
I did a lot of treadmill running this winter, and believe me when I say running 10+ miles on a treadmill is very boring. I'd even venture to say that treadmill running is boring in a convex way, which through Jensen's Inequality, implies that running 10 miles is a lot more boring than running 5 miles twice.
One trick I learned is to divide the workout into smaller pieces, sometimes 400m, sometimes one mile, and not think about all the distance left between you and the finish line.
When you're at mile 15, it's not the thought of the next mile tires you out and exhausts your mettle, rather, it's the fact that you still have 11 whole miles to go until the finish line. If you can, for a while, forget that you have 11 miles to go, and just focus on doing the next mile, you'll be fighting a more familiar problem. You could run a mile in middle school, way before you started marathon training, so you can easily run another mile now.
Rinse and repeat.
This is a little bit harder than it seems and requires a lot of focus, but once you learn to ignore the overall distance and just focus on finishing one mile at-a-time, a 26-mile race becomes a bunch of quick miles you have to knock out.
The importance of resting without stopping.
In order to run very long distances, I've discovered the importance of resting and recharging without stopping. Even if it means running at a walking pace, keep running, albeit slowly, until you feel recharged and can resume your normal pace. The moment you stop moving your feet and hit a standstill, it's hard to start moving your feet again.
I think this is one reason why training is so heavy - marathon runners train upwards of 60 miles/week at peak. With each training run, you're really building a baseline so that running at a slow pace feels the same as resting.
Running is physically taxing but strategically easy.
Running is easy because it does not require a ton of planning. The course is planned out for you, and you don't have to think about strategy. It's the physical equivalent of an infinite loop. One foot in front of the other until you're done. There is very little confusion. The goal is clear, you just have to execute.