Building an aerobic base is CRUCIAL when trying to boost endurance and achieve peak performance as a runner!

If you’ve been following us for a while, you’ve probably heard us talk about the importance of “building an aerobic base.” For today’s newsletter, Coach Sandi Nypaver explains the science behind what exactly happens to your body when you’re building an aerobic base. Through this, we hope you understand why every runner needs to devote a training block to building theirs!

Let’s get started:

 For my runners without a race in the near future, now may be a great time to do an aerobic base phase. During an aerobic base phase, most of your runs will be easy, but you’ll still be making important adaptations.

Adaptation 🥇:

One is the increase in the number and the size of mitochondria you have in your muscle fibers. Mitochondria converts the food we eat. This means they are responsible for turning carbs, fat, and protein into actual, usable energy. The more mitochondria you have, the more energy you can utilize as you run. Mitochondria also help to clear lactate mainly in slow twitch muscle fibers.

Adaptation 🥈:

Another adaptation your body will make during the aerobic base phase is an increase in capillaries. Capillaries are tiny blood vessels that transport blood nutrients and oxygen to cells in your organs and body systems. Increasing the capillaries surrounding your muscle fibers means you can more quickly deliver oxygen and nutrients into your muscles. In other words, your body becomes better or faster at fueling your muscles so they can keep working how you want them to as you run.

Adaptation 🥉:

The third benefit of aerobic based training is an increase in myoglobin, which is a protein that stores oxygen. If oxygen becomes restricted during exercise, myoglobin releases the oxygen to the mitochondria to produce more energy. Another way of putting it is that your myoglobin can serve as your oxygen backup to be used during your race. Easier aerobic running can also strengthen your heart, improve your time to fatigue, and help you better utilize fat as you run.


During an aerobic base phase, the aim is to improve your aerobic system as much as possible before adding in more intense training. If you’re racing anything over a mile, you’re mostly using your aerobic system, which is why aerobic training is so important.

Let’s use a cake as an example. The cake’s icing is your anaerobic system, and the cake itself, so the part made from flour, is your aerobic system. You need both for a good cake, but obviously the cake itself, the aerobic training, makes up most of the cake. The icing, or the anaerobic training, really is the icing on the cake for developing speed.

You still want to add in speed during the aerobic base phase to work on your running economy. I’ll save talking about running economy for another day, but basically adding in things like strides or light fartleks will keep your legs used to running fast during this phase.

In combination with easier running, over time you may see your paces get a little faster while staying at the same effort or heart rate. The short bouts of speed will also help prepare your muscles and tendons for the speed work later on in your training. You can do something like 4-8 x 20 seconds strides every two to three days. Another option is something like strides on Monday and Friday and then have a light fartlek on Wednesday doing 6 x 2 minutes (or something like that).

In conclusion, building an aerobic base isn’t just about improving endurance; it’s about fine-tuning your body for optimal performance. From increasing mitochondria and capillaries to enhancing myoglobin levels, each adaptation works together to make your running more efficient and powerful. So, whether you’re preparing for a race or focusing on long-term fitness, dedicating time to build your aerobic base lays the foundation for success.

Happy running!

  • Coach Sandi Nypaver