The pump, the pipe, and the reservoir

There are three parts to a fluid energy system. There's the pump, which actually pushes (or pulls) the fluid from one point to the other. There's the pipe, which allows for the flow of the fluid energy. Then there's the reservoir where the fluid energy is stored. The human body is not much different, even if the parts are not necessarily tangible.

Our pump is, of course, our heart and lungs. In a fluid system the pump will draw the fluid or energy out of the reservoir. The larger the pump, the more that can be drawn out. If the pump is stronger and more efficient, then it can operate at higher speeds for longer periods. Also, less energy is required to operate and cool the pump so it can run for longer periods. The longer it can run, the more can be drawn from the reservoir at a given time.
A good portion of our training has to do with building the strength of the pump. Marathoners have huge pumps that can be set to run for hours and hours at a time. They can fire up a sprint, but they're not designed to sprint. They burn long and slow, and control the flow through the pipes.

The pipes convey the fluid from the reservoir to wherever it's going. If it's an electrical system, the pipes are actually wires. If it's a fluid system, the pipes are... well, pipes. The bigger the pipe, the more that can flow along the pipe with less resistance. The smaller the pipe, the less that can flow and the higher the resistance.
For a runner, we'd be talking about "fast twitch muscle fibers" and "slow twitch muscle fibers" and such things. It's the efficiency with which we can transfer that energy that our heart and lungs are pumping through our body into actual exertion.
We're also talking about discipline and pace. If we open the pipes too far too fast, we burn out. If we keep them constrained too much for too long, we never hit max performance. Marathons are never won in the first mile, but they can be lost completely in that first mile. Go out too fast and you can sacrifice performance in the final miles. We can always open the pipes up for a sustained release of speed, but then we can close the pipes back up to try and conserve and replenish some of the reservoir. A good portion of the training is also learning how to control the flow through the pipes.

The reservoir simply is what it is. In a fluid system it's the lake, or snow caps, or aquifer. In an electrical system it's the battery. For a runner, it's the... well, it's the "IT". You know, do you have "IT" to make it through to the end? To push through exhaustion? To keep those feet moving even when you can barely focus your eyes? Do you have "IT"? When your lungs are burning and your legs are beginning to tingle and your breath is coming in rasps on every footfall and the line is approaching but not approaching fast enough and the clock is ticking the RELENTLESS CLOCK IS TICKING and the white spots are forming in your field of vision AND THE CLOCK IS STILL TICKING and you just can't go another step but you have to keep pushing for just another few feet and you have to reach deep, deep, deep down into the reserves and pull up just a little bit more of "IT"... yea, that's what "IT" is.

And "IT" is the great mystery of long distance running. Training can expand that reservoir mathematically. But there's something else that adds that little something more beyond the mere edges of the pool of reserve energy. There's that little something else that holds the wellspring of amazing, superhuman energy that separates mere mortals from mere mortals who run... and then those mere mortals who run from mere mortals who are runners... and then those mere mortals who are runners from the gods who walk among us.
Anyone can TALK about running.
In fact, anyone can run.
In fact, anyone can run unfathomable distances--it's in our DNA, we're built for it.
And you know, for that matter, on a full tank of gas anyone can blast off an explosive sprint for a 150 meters or so.
The REAL question is whether or not you can blast off a sprint AFTER running 5 or 10 miles.
Or after running 5 or 10 miles, can you run another 5 or 10 miles? Or, on the brink of exhaustion, can you get up and finish?
That's when you start to find out what IT really looks like.
Not in the fresh legs of a new morning, but in the sweat drenched, slightly parched emptiness that lies just before the edge. When you look deep within and see nothing, but then reach down into that inky darkness and find... yea, there it is.