"I run every session like it's a race, and I've completely plateaued. What am I doing wrong?" Nine times out of ten, the answer is a missing aerobic base.
Base building is the least glamorous and most important part of endurance training, and it is the bit most self-coached women skip. It is not exciting. It does not fill your Strava feed with personal bests. But it is the foundation every bit of your speed is built on, and without it you stall.
Here is what an aerobic base actually is, why building one makes you faster, and exactly how to do it without either boring yourself rigid or secretly running every "easy" day too hard. Which, let me guess, is what you are doing now.
Your aerobic system is what powers almost everything in endurance, from a parkrun to an Ironman. The bigger and more efficient it is, the faster you can go before you tip into the red, and the longer you can hold it.
You build that engine with lots of easy, aerobic work. Not with constant hard sessions. The women who plateau are usually the ones grinding every run at a moderately hard effort, which is just hard enough to leave them tired and just easy enough to never force real adaptation. The fix is to go properly easy, most of the time, so you can go properly hard when it counts.
What Builds When You Go Easy
Easy running feels like it should not be doing much. It is doing a huge amount. At low intensity, your body makes a string of quiet adaptations that nothing else builds as well.
- More and bigger mitochondria, the little engines in your muscles that produce energy.
- More capillaries, so more blood and oxygen reach your working muscles.
- A stronger heart that pumps more blood with each beat.
- Better fat burning, so you spare your limited carbohydrate stores for later.
- Tougher tendons, bones and connective tissue, because they adapt slowly and easy volume gives them time.
That last point matters enormously for women. Your heart and lungs adapt quickly, but your tissues adapt slowly. Easy aerobic volume is how you build the durability to handle harder training later without falling apart. Skip the base and you are building speed on top of a body that is not ready for it.
The 80/20 Principle
The simplest, most evidence-supported way to structure your training is often called polarised, or the 80/20 approach. It is exactly what it sounds like.
Most of the week is easy, conversational, build-the-engine work. A smaller slice is properly hard, quality work that lifts your ceiling. What you avoid is the grey zone in the middle, the moderately-hard effort that feels productive but mostly just makes you tired.
This is how most of the best endurance athletes in the world train, and it works just as well for a busy age-grouper as it does for an elite. The ratio matters more than the exact number.
The hardest part of 80/20 is the 80. Running easy feels too easy, especially if you are fit and competitive. But that is the point. The easy days make the hard days possible, and the hard days are where the speed comes from.
Your threshold pace in, your seven training paces out, in minutes per km or per mile.
How to Know You're Actually Going Easy
This is where most women get it wrong, so let me be specific. Easy should feel almost annoyingly gentle. Three ways to check.
- The talk test. You should be able to hold a full conversation in complete sentences. If you can only get a few words out, you are going too hard.
- Heart rate. If you train with it, your easy runs should sit in the low aerobic zone, well below your threshold. For many women that feels shockingly slow at first.
- Breathing. Nose-breathing comfortable, no gasping. You finish feeling you could have kept going for ages.
Yes, it will feel slow. Yes, people might overtake you. Let them. In a few weeks, that same easy effort will produce a faster pace, because your engine has grown. That is the whole point, and it is deeply satisfying when you see it happen.
The Mistakes That Wreck a Base Phase
The classic. Your easy run creeps up to moderate because moderate feels better and more productive. Over a week, all your runs blur into the same medium effort.
Fix: hold yourself back deliberately. Use the talk test every run. If in doubt, slow down.
The flip side. When your quality sessions come around, you are too tired from the grey-zone easy days to hit them properly. So the hard days are not hard either.
Fix: protect your easy days so your hard sessions can be genuinely hard. Polarise.
Base building is a slow burn. The gains are real but they take weeks, so impatient athletes bail and go back to smashing every run.
Fix: give it time. A base phase of several weeks pays off for the whole season that follows.
More easy volume still needs fuelling. Trying to build a base while eating too little stalls your adaptations and raises your injury risk.
Fix: eat enough to support the training. Base building is not the time to also diet.
How Long Should a Base Phase Be?
It depends on your starting point and your goal, but think in terms of weeks and months, not days. A dedicated base phase of several weeks early in a training block sets you up for the sharper, faster work later. For a big goal like a marathon or a 70.3, you might spend a good chunk of the early plan mostly building base before you layer in race-specific intensity.
And base work never fully stops. Even when you are deep in a race build with lots of quality, the bulk of your week stays easy aerobic running. You do not build a base once and move on. You keep the engine running underneath everything else, all year.
Does the 20% Mean I Never Go Hard?
Not at all. The opposite. A proper base phase earns you the right to go genuinely hard in your quality sessions, because you are recovered enough to hit them. The hard 20% might be threshold work, intervals, hills or strides, depending on where you are in your plan. The base is not instead of speed. It is what makes your speed work actually work. Think of it as sharpening a well-made knife rather than hacking away with a blunt one.
A Note for Triathletes
Everything here applies across swim, bike and run. The 80/20 idea holds for your whole training load, not just your running. Most of your riding and swimming should also be easy aerobic work, with a smaller slice of quality. The trap for triathletes is turning every session in all three sports into a moderate grind, which stacks up into chronic fatigue fast. Keep the easy stuff easy across all three, and your hard sessions, and your race, will thank you.
What a Base Week Looks Like
Theory is lovely, but it helps to see the shape. Here is roughly how a base week might run for a runner. Adjust the number of days to your life and experience.
- Three to five easy aerobic runs, all at conversational pace, making up the bulk of your week.
- One of those runs a bit longer, building gradually, still easy.
- One session with a little quality, for example some strides or a short, controlled faster block, kept to that 20%.
- Two strength sessions, because strength is part of building a durable base.
- At least one full rest or very easy day.
Notice how little genuinely hard running there is. That is deliberate. In a base phase, the easy volume is the work, and the small amount of quality just keeps your legs ticking over. The intensity ramps up later, once the engine is built.
How You'll Know It's Working
Base building is a slow burn, so it helps to know what progress looks like, because it is not a weekly personal best. The signs are quieter and more satisfying.
Your easy pace gets faster at the same heart rate and the same easy effort. This is the big one. You are running quicker for free.
Runs that used to feel like a slog start to feel comfortable.
You recover faster between sessions.
Your resting heart rate may drift down over time.
You get to the end of your long runs feeling you had more in the tank.
Keep a simple log and look back over a few weeks rather than day to day. The trend is where the story is. When you see your easy pace quietly improving while your effort stays the same, that is your base doing its job, and it is a brilliant feeling.
Why This Is So Hard for Driven Women
Let me name the real obstacle, because it is rarely the training itself. The hardest part of base building is psychological. If you are an ambitious woman juggling a lot, training time is precious, and running slowly can feel like you are not trying hard enough, not making it count. Every instinct says push. Add in a culture that celebrates the grind and a Strava feed full of other people's hard efforts, and holding yourself back feels almost wrong.
Here is the reframe. Running easy is not slacking. It is the disciplined, grown-up choice that makes everything else work. It takes more self-control to hold back than to hammer away. The women who improve year after year are the ones who get comfortable going slow on the slow days, precisely so they can go fast on the fast days. Think of your easy runs as deposits in a bank account you spend on race day. Boring, patient, and exactly how fitness is built. Brick by brick, remember. And the lovely thing is, once you feel your easy pace quietly getting quicker for the same effort, the patience stops feeling like a sacrifice and starts feeling like the smartest thing you have ever done with your training. That moment, when the slow running suddenly pays off and your easy pace drops without any extra effort, is one of the most satisfying things in endurance sport, and it is waiting for every woman who is willing to go slow enough, for long enough, to earn it. So be patient with it. The base you build now is the base your whole season stands on.
The Short Version
Your aerobic base is your engine, and speed is built on top of it. Skip it and you plateau.
Easy aerobic work builds mitochondria, capillaries, a stronger heart, better fat burning and tougher tissues.
Aim for roughly 80% easy, 20% genuinely hard. Avoid the moderately-hard grey zone.
Easy means conversational. Use the talk test, heart rate and your breathing to keep it honest.
Give a base phase several weeks, fuel it properly, and keep easy volume underneath everything all year.
The base is what lets your hard sessions be hard enough to actually make you faster.
Build the engine first. It is the least exciting thing you will do all season, and the reason everything else works. Brick by brick.
- You run every session hard and you've completely plateaued
- You find easy running almost impossible to do slowly enough
- You want a plan with proper structure, not just "run more"
- You want a coach who builds your engine before chasing your splits
If you ticked two or more, this is exactly how I build my athletes.
Apply to work with me →If you want training that builds your base properly and then layers speed on top, that is what I do.
Female-specific endurance coaching with structure that actually progresses you. Easy days easy, hard days hard, built around your real life.
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Apply to work with me →Frankie x