In Part 4.0, we discussed how to get started with running: take it easy. I repeat — take it easy — if that is the only message you took away from the post.
In this post, I will go into the science behind running: how it actually works, and what happens in and to your body when you run. Understanding this is a crucial step in giving meaning to each of your training sessions, even if they feel “too easy,” as recommended in Part 4.0. This will be a longer post and may take some time to fully digest.
As discussed at the end of Part 3.0, our bodies use energy from multiple sources. The balance of how much energy comes from each source depends largely on the type and intensity of activity being performed. I will expand on this in this post.
There are three main energy sources that the body uses:
- Blood glucose (sugar currently circulating in the bloodstream).
This may come from food that you have recently consumed, such as energy gels and drinks. - Glycogen stored in the liver and muscles.
Recall that glycogen is a stored form of carbohydrate. Glycogen can be broken down very rapidly to release glucose for fast energy production. The body can use glucose both aerobically (with oxygen) and anaerobically (without oxygen). The faster and harder we run — such as during an all-out sprint — the more the body relies on anaerobic glucose metabolism.
However, this is not sustainable for long periods of time. Anaerobic metabolism produces byproducts such as lactate and hydrogen ions, which eventually accumulate faster than the body can clear them. When this happens, muscles begin to burn, fatigue rapidly, and eventually stop functioning effectively.
- Fat.
Recall that most of us carry enough fat to provide energy for days, weeks, or even months. Practically speaking, even lean elite runners carry enough fat that they will never truly run out of it during a race.
Fat is an extremely efficient form of stored energy, recall:
- 1 gram of fat = 9 calories
- 1 gram of carbohydrate or protein = 4 calories
However, there is a catch.
Fat is broken down much more slowly than carbohydrates. This means that the faster you run, the less your body can rely on fat for fuel. During an all-out sprint, virtually no fat is used. During an easy jog or walk, the body relies primarily on fat metabolism. At all effort levels in between, the body uses a mixture of carbohydrates and fat.
The body is smart. It will always try to use the most efficient energy system available for the demands being placed upon it. The body is fundamentally an energy-conserving machine, not an energy-wasting machine — which is one reason why weight loss can be so difficult for many people — the body wants to fight against weight loss when possible.
The second issue is that many untrained individuals have lost much of their ability to efficiently utilize fat as an energy source. Modern lifestyles promote sitting and constant food availability. Our bodies are often bathed in a near-continuous stream of calories.
This creates several problems:
- Many people begin to feel hungry as soon as blood glucose levels start to fall. This triggers eating, which causes another glucose spike. As glucose falls again, the cycle repeats.
- Constant eating and snacking prevent glycogen depletion, meaning the body rarely needs to significantly tap into fat stores for energy.
- Most people become relatively efficient at using glucose, but inefficient at using fat.
For example:
- Most people can sprint briefly to catch a bus.
- Most untrained people cannot comfortably jog for over an hour.
We also need to remember that fat metabolism requires oxygen. Glucose, however, can be used both aerobically and anaerobically.
The optimal way to race a given distance is to use fat and glycogen in the right proportions, ideally reaching the finish line just as glycogen stores become nearly depleted.
If glycogen runs out too early, the runner “hits the wall.” Pace slows dramatically because the body is now forced to rely primarily on fat metabolism alone. This feels like wanting to run faster, with the heart still feeling relatively okay, but the legs simply refusing to respond. It almost feels like running with brakes on.
On the other hand, if you finish a race with large glycogen reserves remaining, you likely ran too conservatively and could have maintained a faster pace (if your legs could handle the pounding, that is).
Learning how to manage these energy systems is one of the keys to becoming an efficient runner.
To become a strong endurance runner, we need to train the body to do several things:
- Improve fat utilization.
The body must become highly efficient at using fat for fuel. This conserves glycogen stores and improves endurance. - Build a strong cardiovascular system.
The heart and blood vessels must efficiently deliver oxygen to working muscles. - Strengthen the musculoskeletal system.
Muscles, tendons, ligaments, and bones must become durable enough to tolerate repeated impact.
If your heart is strong but your legs are weak, it is like putting a Ferrari engine into a car with weak Toyota wheels. The engine may be powerful, but eventually the wheels fall off.
This, by the way, is essentially what happened to me during my first marathon.
This is where the concept of 80/20 running comes in.
80/20 running is now a well-established concept in endurance sports, including:
- running,
- cycling,
- swimming,
- rowing,
- cross-country skiing,
and more.
I will not go deeply into the history of how 80/20 training was discovered, but in summary, it emerged over time as training methods evolved. The methods used by successful endurance athletes survived and spread, while less effective approaches gradually disappeared.
Today, nearly all elite endurance athletes train using some variation of this principle, and research also supports it as one of the most effective approaches for recreational runners.
So what is 80/20 running?
Approximately:
- 80% of running should be easy,
- and only about 20% should be hard.
Easy running means conversational pace running.
Hard running means efforts faster than conversational pace.
Most runners who think they are doing 80/20 are actually not. Beginners especially tend to:
- run their easy days too hard,
- and their hard days too easy.
They end up constantly stuck in the “grey zone” — training moderately hard all the time, which limits progression and increases injury risk.
Many beginners effectively train closer to:
- 50/50,
- or even 20/80.
One major issue is that many beginners do not yet have a true “easy running pace.” As soon as they start jogging, breathing becomes laboured and the effort becomes hard.
This is completely normal.
That is why I recommend beginners start with walk–jog intervals as discussed in Part 4.0. The effort must stay easy while fitness catches up.
Most beginners should avoid dedicated speed work until they can comfortably recover from easy running 4–6 days per week. Rushing into hard training usually leads to injury.
So why does 80/20 work?
First, it allows us to run more.
Running volume is one of the primary drivers of endurance adaptation, and easy running allows us to accumulate far more training time safely and consistently.
When we run consistently:
- the heart becomes stronger,
- blood vessels become more efficient,
- muscles adapt,
- tendons and ligaments strengthen,
- lungs improve,
- and even cellular-level changes occur.
Easy aerobic running specifically stimulates mitochondrial growth within muscles.
Mitochondria are essentially tiny energy factories inside cells. More mitochondria improve the body’s ability to use oxygen and fat efficiently for energy production.
This makes running feel easier and more economical over time.
Easy running is therefore not “junk miles.” In many ways, it is the foundation of endurance performance.
80/20 running also reduces injuries.
Hard running places significant stress on:
- joints,
- muscles,
- tendons,
- and connective tissues.
Most hard sessions require multiple days of recovery. If every session is hard, injury eventually becomes inevitable.
Easy running allows the body time to recover and adapt.
Remember:
Consistently good beats occasionally great every single time.
Over time, along with strength training (more on this in another post), repeated manageable stress strengthens the body. Muscles, tendons, ligaments, and bones gradually become more resilient.
Eventually, the Toyota wheels become Ferrari wheels.
80/20 is also psychologically sustainable.
Most people associate running with suffering because nearly all of their past running experiences were hard:
- sprinting in PE class,
- punishment laps,
- or exhausting attempts to “get fit.”
Easy running changes this relationship.
If 80% of your training feels enjoyable and manageable, the remaining 20% of harder sessions becomes much easier to tolerate — and even exciting.
Consistency becomes far more achievable.
80/20 also improves running economy.
Running economy refers to how efficiently you use energy while running:
- using less energy for the same speed,
or: - producing more speed with the same energy.
You want to become like a fuel-efficient hybrid car during distance running, not a Ferrari that burns through fuel rapidly.
One of the best ways to improve running economy is simply to run more — and 80/20 allows you to safely run more.
Every step helps refine coordination between:
- the brain,
- muscles,
- tendons,
- and movement patterns.
Over time, the body naturally learns to move more efficiently.
How about the 20% of hard running?
Honestly, this is far less important than most beginners think.
When we say 80/20, we mean:
at least 80% easy.
You could improve for a very long time doing:
- 90% easy,
- 95% easy,
- or even 100% easy running.
Hard running becomes increasingly important only after a strong aerobic foundation has been built.
Speed work can improve:
- lactate clearance,
- running economy,
- neuromuscular adaptations,
- and top-end performance.
But speed work must be layered on top of a strong aerobic base. Without that foundation, harder training mainly increases injury risk without providing maximal benefit.
There are many types of harder workouts, including:
- Tempo Runs
- Threshold Runs
- Critical Velocity Runs
- Track Intervals
- Hill Repeats
- Fartlek Runs
- and others
These terms may sound intimidating right now, and that is okay.
At this stage, I do not want you to become distracted by the 20% and lose focus on the 80%.
If you have not yet learned to enjoy easy running, then hard sessions will probably feel miserable.
For beginners, easy conversational running is more than enough to make enormous progress.
Even if easy running was all you did for several years, you could still:
- become dramatically faster,
- improve endurance significantly,
- and complete long races such as marathons.
To quote my mentor Kate Dilworth:
“Go slow to go fast.”
I would also add:
“Go slow to go far.”
It is very difficult to “go hard to go far” in distance running.
What I have written so far is enough to work on for potentially years. There is a wealth of excellent information available if you eventually want to learn more about advanced speed training, but please do not rush into it — that is doctor’s advice.
I will discuss speed work in much more detail in future posts.
Once again, just like in Part 4.0, if there is only one takeaway from this post, it is this: take it easy. I used two entire posts just to tell you to take it easy — that is how important it is.
In Part 4.2, I will introduce some running gear and gadgets — including shoes — that you may or may not need before getting started.
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