Most people think aerobic training is the thing that gets you through a long run. That is the smallest part of what it does.
Your aerobic system produces energy when you are not under pressure, which is almost all of the time. Walking up the stairs at work. Getting through Thursday afternoon without needing coffee. If it is underbuilt you feel it long before you feel it in a race. The pattern I see most is people who train reasonably hard three or four times a week and still feel flat all day.
What the aerobic system actually is
- Mitochondria. Tiny engines inside your muscle cells that turn fuel and oxygen into energy. More of them means more energy at the same effort.
- Capillaries. The smallest blood vessels, the ones that deliver oxygen into the muscle and carry waste away. The last hundred metres of the delivery route. You can have a strong heart, but if that network is thin the oxygen does not arrive.
- The ability to burn fat. At easy efforts a well built aerobic system runs largely on fat, which you have in near unlimited supply. An underbuilt one reaches for carbohydrate much earlier, and carbohydrate runs out.
One thing to be clear about, because it gets sold badly. Burning fat is not the same as losing fat. Fat burning is about which fuel you reach for while moving. Fat loss is about energy balance across the week, food included. You can get much better at burning fat and not lose a kilo.
The everyday win is better anyway. A body that runs on fat at easy efforts is not constantly demanding sugar, which shows up as steadier energy and fewer crashes.
Why aerobic training decides your daily energy
Every task has an energy cost. If your aerobic system is underbuilt, ordinary tasks sit at a higher percentage of what you have got, so you work harder than the person next to you to do the same thing. Much of what people call being tired all the time is that.
A 2018 study followed 122,007 people after treadmill testing. Compared with the fittest group, the risk of dying stacked up like this:
Being unfit carried a bigger risk than smoking. They found no upper limit of benefit either. Fitter kept being better, all the way up.
The signs your base is underbuilt
You will see the term “aerobic deficiency syndrome” in low heart rate training circles. Straight with you, from both sides. It is not a recognised medical diagnosis, it has no clinical criteria, and Cliff Pittman of CTS is right to say so publicly. I still use it with clients, because it is a good thinking tool. It gives someone a way to understand why they feel flat and why more effort has not fixed it.
Professor Tim Noakes put forward the central governor model, the idea that the brain limits how hard you are allowed to work based on what it senses. When glycogen, your stored carbohydrate, runs low, the brain reads an energy threat and turns up your perceived effort. The pace has not changed, it just feels harder. The model is debated, but the practical part holds: leaning on carbohydrate you do not have much of makes everything feel heavier than it should.
- You cannot hold a conversation on an easy run. Short, clipped sentences means it is not easy.
- Every session feels the same. Moderately hard. This is the grey zone, where most recreational runners live. Hard enough to build fatigue, not hard enough to build much else.
- Your pace drifts in the back half. Same effort, same heart rate, slower.
- You are not improving at the same effort. Same route, same heart rate. If the pace has not moved in months, something is not developing.
- The afternoon crash. Not always training related and worth raising with your doctor, but a weak base contributes.
- Easy sessions wreck you. A 40 minute run should not cost you the evening. If it does, that run was not easy for your system, whatever your watch said.
That last one matters most. When it happens I do not tell the client to push through. I take them lower, below their MAF number, into a zone I call LAHR, low aerobic heart rate, usually 10 to 20 beats under MAF. It feels ridiculous and most people have to walk the hills. But this is where the system rebuilds instead of absorbing stress it cannot recover from. Once easy stops costing them the evening, the number comes back up.
One sign proves nothing. Three or four together is a pattern.
Where the internet gets this wrong
I use low heart rate work and I will keep doing it. But the Zone 2 conversation online has run past the evidence, so here is the honest version.
- Zone 2 is not magic. A 2025 Sports Medicine review, Much Ado About Zone 2, tested the claim that easy work is the best intensity for building the engines and the fat burning machinery. The answer was no, and swapping all your harder work for Zone 2 may leave you with less health benefit.
- But easy work builds what hard work cannot. A second 2025 paper pooled around 6,000 people and found easy endurance training grew the capillaries better than harder training did.
- The engines grow either way. Easy and hard built them by a similar amount, hard intervals just faster.
Both camps are half right. Easy volume builds the plumbing, intensity builds the engine faster, and you need both. Build the base first, then add intensity onto it. Never going hard is its own ceiling. Going hard all the time is the commoner mistake.
How you sequence that is a subject on its own. I have written it up in training in blocks, and why the order of your training decides the result.
What to actually do
Make your easy runs genuinely easy. Easy enough that it feels like cheating. If easy still costs you your evening, go to LAHR until it does not.
Accumulate across the whole week. The pattern I see constantly is someone who does nothing Monday to Friday because work is flat out, then makes up for it with a big weekend. They are not lazy, the week got away from them. But the body does not grade intentions. It responds to how often you give it the signal.
So use micro sessions. With beginners and anyone rebuilding a base I lean on these heavily. Twenty minutes before work, a 25 minute walk at lunch. A 20 minute easy session tells your body to build capillaries. Five times a week tells it five times. Nothing Monday to Friday tells it nothing, however big Saturday is. Almost nobody has 90 minutes on a Tuesday. Nearly everybody can find 20.
Give it twelve weeks, and test the same way every six. Same route, same effort, compare the pace.
If you have trained consistently for a year and feel no better in daily life than when you started, the problem is almost never how hard you are trying. It is the structure, and structure is fixable. That is what coaching with MFHP Adventures is built around.