What if I told you that by running at a lower intensity, you could boost your running efficiency by 600%?

You’d think I was lying.

Well, I’m not. That’s exactly what happens when we run at the right intensity. When we’re burning sugars anaerobically, the sugar only gets partially processed by the cell, and out comes lactate. But when we burn them aerobically, that lactate goes through another process: The Krebs Cycle.

One of the reasons why we bonk during our runs—and why we hit The Wall during the marathon—is because we don’t understand this little process. We think that burning sugars anaerobically just means that we get out of breath and that we produce lactate. But what it also means is that we can only get one-sixth of the total energy out of that sugar molecule.

Let’s just say that isn’t the best way to slog through a marathon.

But to see how to change that we have to go inside the cell.

Inside of the cell, the glucose that is supplied by the bloodstream gets converted into the cell’s main energy source, adenosine triphosphate, or ATP. When there’s no oxygen to be had, the cell gets 5 ATP out of the glucose molecule, and out comes a highly acidic substance called lactate. By making our body more acidic, lactate threatens our survival.

So, after a few seconds of exercising anaerobically, the brain sends signals forcing the body to stop working: if not, the cells will begin falling apart. So the body has to figure out a way to get the lactate out. And it does that by converting it into a molecule called pyruvate, but it can only do that in the presence of oxygen. This conversion process costs the cell 2 ATP.

However, instead of converting glucose to lactate, and then lactate to pyruvate, there is a shortcut: use oxygen to break down the glucose molecule, and you go straight to pyruvate. In the human body, when there’s a solid oxygen presence, the body prefers to convert fats to sugars, and use those as energy. In other words, the body’s long-term energy sources activate when there’s oxygen.

Pyruvate, as opposed to lactate, can go through the Krebs Cycle, a process also known as aerobic respiration. Pyruvate gets paired with oxygen inside of the mitochondria—the little aerobic energy factories inside the cell—and generates 25 more ATP, for a total of 30. That’s 6 times as much energy immediately available, than if glucose would have gone through anaerobic respiration.

The by-product of the Krebs Cycle, of course, is CO2. Having turned lactate into CO2, the acid is now in a form that the body can expel (through the lungs) and bring its pH back up to a manageable level. It’s important to mention that the initial reason anaerobic exercise stops isn’t due to a lack of oxygen; it’s because of the lowering pH. Although a lack of oxygen will kill us if the body continues to acidify at that rate, we’ll be incapacitated long before our oxygen runs out.

But the reason that we turn glucose into lactate instead of pyruvate is because we go too hard.

You can think of anaerobic respiration as sort of the body’s emergency system. Because it’s far more dangerous to use it (but also it doesn’t need oxygen to produce energy), the body only shifts gears and begins doing mostly anaerobic respiration when it feels that there’s a crisis. So, if you, at any point during your run, would describe the feeling as going hard, you’re probably doing a significant amount of anaerobic respiration.

There’s another tidbit of wisdom hidden here: because anaerobic respiration is a crisis-response mechanism, it is deeply tied into the stress response. That means that if you’re stressed, your body will be doing an increased amount of anaerobic respiration, even when you’re completely still. Your body will be needlessly acidifying—and all of your tissues will be that much weaker for it.

On the one hand, this is a cautionary tale against stress: not only will it make your body underperform in the long term (soaking your tissues in acid isn’t that good for their longevity, I’ve heard), but it also illustrates the necessity of slowing down the long runs to actually train the aerobic systems that were intended to be exercised. In Born to Run, Chris Mcdougall quotes running coach Ken Mierke, who says:

“Nearly all runners do their slow runs too fast and their fast runs too slow. So they’re just training their bodies to burn sugar, which is the last thing a distance runner wants. The way to activate your fat-burning furnace is by staying below your aerobic threshold—your hard-breathing point—during your endurance runs.”

But even that may not be enough for the competitive or busy athlete: in order to get the most out of those long, slow runs, it’s important to remain unstressed. If you’re stressed, even those “aerobic” runs will be a lot more anaerobic than you intended them to be. This, by the way, is the first stage of overtraining.

Slowing down for our long runs isn’t selling ourselves short. Taking rest days doesn’t amount to “missing” a workout. There are a lot of different parts of the body that we have to learn to manage, and unless we want our athletic ability to have a shorter shelf-life, we’ll need to train with a little bit more humility, and a little bit more vision.

It’s no coincidence that the pun “training for the long run” works so well here. It’s a telling symmetry of the system that the long run itself reduces stress and prolongs our well-being. Let’s take heed of that.