Taking all these hard science classes recently has made me think about how so much of what makes a great endurance athlete happens at the microscopic level. Yes, when we get "in shape" we might lose a few pounds - gain some muscle etc - but what really matters is happening within our cells. All of our cells are dependent on the following chemical reaction C6H12O6 + 6O2 > 6CO2 + 6H2O + Energy. Simple aerobic respiration - and we literally couldn't do anything without it.
As you probably know the 6CO2 + 6H2O are just byproducts - which is helpful since the process that started made aerobic respiration possible, photosynthesis, only needs 6H2O and 6CO2 + light in order to make 6O2 + C6H12O6. Photosynthesis and aerobic respiration - the real yin and yang of life.
Anyways, aerobic respiration occurs in a specific place in our cells. It starts out in the cytoplasm (space inbetween organelles) but the rubber really hits the road in the organelles called mitochondria. You probably remember mitochondria as being called the "power houses" of our cells.
But, let's back up a second. We know that aerobic respiration results in energy, but how does the cell actually store and release that energy? ATP - basically a nitrogen base, a 5 carbon sugar, and 3 phosphate groups. The phosphate groups are key - when one breaks off you get a little energy and ADP. Aerobic respiration turns 36 ADP into 36 ATP - so that they are ready to go around the cell and lend energy to whatever reaction/process needs it (e.g. contracting a muscle fiber so that we can move forward as quickly as possible).
2 ATP are generated in the cytoplasm during aerobic respiration - but 34 are generated within the mitochondria. There's an interesting theory that mitochondria weren't originally organelles, but were actually bacteria. These bacteria entered into a mutualistic relationship with cells - where each organism got something it needed from the other. They were having such a groovy time together that the cell said - "hey mitochondria! why don't you just move into my house?" The mitochondria said "hey, why not?" What followed is the most sucessful shackup of all time. And if our cells ever throw mitochondria's stuff out onto the street - we're pretty much screwed.
When we're out of shape our cells have a small number of mitochondria - and they're lazy. They don't produce as much ATP as they do when we're in shape. So, the most important thing that happens when we exercise regularly is that our cells get more mitochondria and those mitochondria are more efficient.
The other important process that happens when we're getting in shape is that our body produces more red blood cells. That's why some endurance athletes cheat by blood doping (injecting saved red blood cells before an important race) or take EPO (a hormone that controls the process of producing red blood cells). The reason why this is important goes back to aerobic respiration - we need to get those 6O2's from somewhere.
Depending on the distance, the third important part of getting in shape is training your body to store a lot of glycogen, which is broken into glucose (C6H12O6) for aerobic respiration. Yes, your body can use lipids to get the glucose, but it's much less efficient.
What's the main take away? The most important thing to be in shape is to have cells that can do a large amount of aerobic respiration - by having a lot of the ingredients necessary readily available (glucose and oxygen) and by having cells that include a large number of highly efficient mitochondria.
How to do it? That's for another post. Although I will say that looking at getting into shape at the cellular level shows why there aren't any shortcuts to good performances. Training consistency and volume might seem boring - but our cells need enough regular stress so that they know to produce those high-performing mitochondria.