If you want to run a 15 minute 5k, common sense would tell you that you need to run as close to 15 minute 5k pace as possible to improve, so that would mean running 3 min/km pace for as long as possible and as often as you can. Whilst I 100% agree this seems logical, it is not the case. This is because running at such high intensity frequently will only lead to overtraining, burn out and unfortunately, most probably an injury.

The Paradox of Running at a Slower Pace

It is a paradox that to run fast you must run slow – or more like, running slow can make you run faster. A common thought among all runners is that ‘I must run as hard as I can to get fitter and faster, quicker’.

Running slow though will allow you to increase your weekly mileage and develop your aerobic system and slow twitch muscle fibres further without overexerting your body. In addition, these slower paced runs can function as active recovery from the harder workouts you have scheduled throughout the week. You won’t be able to run as fast in these harder workouts if you are doing your easy runs ‘hard’.

So How Do Easy Runs Make You Faster?

By doing easy runs, you train your aerobic system – that is the system within your body that uses oxygen to respire and create energy that your muscles use to contract and make you run fast. When you run at higher intensities, your anaerobic system comes into play and that means you can’t train your aerobic system as efficiently.

Training the aerobic system is key to running faster over longer distances from the mile upwards. In easy and long runs, you want to target the aerobic system so that you can develop it further. Maybe you’ll understand just how important the aerobic system is if you see how the percentage of energy we generate from our aerobic system changes as the distance we run gets longer:

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Energy contribution from aerobic and anaerobic system depending on your race event

As you can see, even running 1 mile is 80% aerobic which emphasises the importance of the aerobic system for the middle distance events.

What is the Aerobic System?

The aerobic system is the system in our body that releases energy with oxygen. ‘Aerobic’ means with oxygen and ‘anaerobic’ means without oxygen. By training your aerobic system, you are training your body to release energy more efficiently with oxygen. If you push your body too hard, it will require more energy and so it will begin to use the anaerobic system to meet the demands of higher intensity exercise.

Utilising your aerobic system whilst running is known as aerobic glycolysis – glycolysis being the process in which glucose is broken down by enzymes to release energy. Whilst running at an easy pace, your muscles receive enough oxygen through normal breathing to use for aerobic glycolysis to release energy. By training this system, you will improve your body’s capabilities to effectively transport and utilise oxygen for aerobic glycolysis allowing you to run faster.

Physiological Developments from Training the Aerobic System

So we know that training the aerobic system will help to make you run faster in the long run, but what physiological adaptations occur as a result of doing easy runs at a pace that is targeted at the aerobic system?

  • Capillary development
  • Increased myoglobin content of muscle fibres
  • Mitochondrial development

Capillary Development

Capillaries are the smallest blood vessel within your body and help to transport oxygen and nutrients to where they are required within the body whilst flushing waste products such as carbon dioxide and lactic acid out of the body.

Aerobic training helps to increase the number of capillaries within each muscle fibre.

This means your body has increased the number of pathways oxygen and nutrients can take to get to your working muscles. Therefore, you can get oxygen and glucose to your working muscles quicker and remove waste products quicker. Your body becomes more efficient as this process allowing you to run faster.

Increased Myoglobin Content of Muscle Fibres

Myoglobin is a red protein containing haem, an iron-containing compound which carries and stores oxygen in its cells. It is very structurally similar to haemoglobin, a protein which carries oxygen in the blood.

When exercising, working out or running at high intensities, your body requires more oxygen. If you have more myoglobin this means more oxygen can be released to your muscles to produce energy for muscular contractions, allowing you to run faster when your body is aerobically stressed.

Mitochondrial Development

Mitochondria are those tiny little powerhouse organelles that produce energy via respiration. All energy (ATP) in your body is produced at your mitochondria. Using oxygen, mitochondria can break down various organic compounds such as carbohydrates, fats and proteins to produce energy.

Aerobic training helps to increase the number of mitochondria within your muscles fibres and the size of your mitochondria. This means more energy can be generated during training allowing you to run faster and longer.

Can I Run Faster for Better Results?

You would think so, but this actually isn’t the case.


Running faster will result in decreased aerobic returns and will increase the risk of injury and burnout by overtraining. In addition, by running faster you begin to train and utilise the anaerobic system which is not necessarily a bad thing, but as you can see from the chart above, compared to the aerobic system, the anaerobic system is hardly used – especially in the longer events.

What Pace Should I Do Easy Runs At?

Thanks to scientific research, we can better identify the ideal pace to run at in order to increase capillary density, myoglobin content in muscle fibres and mitochondrial development.

Research has shown that optimum VO₂ max for the three physiological adaptations is:

This equates to 50 – 75% of your 5k pace to best stimulate capillary development and 65 – 75% of your 5k pace to best stimulate mitochondrial development. It is due to these margins that there is little benefit to going faster on an easy run, even if you feel like it.

To calculate the 5k pace you need to run at to hit these margins for optimal physiological development you can:

  1. Figure out what percentage of your 5k pace you want to run e.g. I want to run 70% of my 5k pace for my easy runs.
  2. Take away the percentage pace away from 100 i.e. 100 – 70 = 30.
  3. Add your result to the number 100 e.g. 30 + 100 = 130%.
  4. Convert your percentage into a decimal e.g. 130% = 1.3
  5. Take your 5k pb in seconds and divide by 5 to get your average min/km pace e.g. 1000 / 5 = 200 seconds per km = 3:20 min/km
  6. Multiply your pace in seconds by your multiplier i.e. 1.3 x 200 = 260 seconds
  7. Convert your answer back into minutes i.e. 260 seconds = 4 minutes 20 seconds

The Benefit of Taking Easy Runs Easy

By not dashing off and doing another run like you have to be drop dead tired right at the end you reduce the stress placed on muscles, tendons, bones and ligaments which will help to facilitate a better training session in the next day or two than if you ran hard.

Easy days can function as active recovery and prevent a muscle from becoming overused by not overworking it constantly. However, this obviously will not work if you run too fast or overexert yourself because running at a ‘hard’ pace will cause microtears to form within your muscles causing muscle soreness making the consecutive days of training planned more difficult.

How do you dictate the pace of your easy runs? Let me know down below!