The Working Form · No. 03 ·
Conservatively underloaded
Back running after a long stint off, I went out for what I would have called a moderate 4 miles. It felt like the kind of run I have done hundreds of times, nothing about it felt hard while I was in it, and then I looked at the data afterwards and every single mile was zone 4. Not the last mile, not a hill in the middle, the whole thing. I didn't feel good after it either, and I spent most of that evening trying to work out how a run I would have honestly described as easy came back as a threshold effort.
Let me take a step back and explain how I got there.
Heart rate zones are just a way of slicing a session by how hard the cardiovascular system is working. Zone coverage is the share of the run that landed in each of those bands, so a report that says every mile was zone 4 is a coverage report, not a vibes check. Zone 2 is the easy aerobic band. The accepted field test, when you don't have a lab, is whether you can still hold a conversation at that pace. If you can talk in full sentences, you are roughly there. If you can't, you have already left it.
The model I imported
I have some experience in the gym working on strength and hypertrophy, so progressive overload isn't an abstract idea for me. I have spent far more time thinking about that model than I have about heart rate, and I carried the lifting model of adaptation straight into endurance without ever really inspecting it. Under a barbell the logic is clean, you expose yourself to a stimulus, you recover from it, and after enough repeat exposures that same stimulus gets easier so you add load. I assumed running worked the same way, do the run, whatever it happens to feel like, and eventually the run gets easier. Under that assumption the zone genuinely didn't matter to me, it was just a number describing how hard a particular day had been, and as long as I got the miles or the ride done I was successfully working toward the goal. That was wrong.
So I went back to first principles and asked myself,
If repeat exposure to a stimulus is what drives the adaptation in the gym, what exactly is the stimulus I am repeating on a run?
I didn't have an answer, which is usually the sign that I have been running on an assumption instead of an understanding.
What zone 2 actually means
That question sent me into zone 2 podcasts and videos, and after listening to Dr. Íñigo San Millán on The Proof it became pretty clear that I had been treating aerobic exercise mostly wrong. His points, as I understood them, were as follows.
- Zone 2 has an actual physiological definition rather than a perceptual one, it is the highest intensity at which blood lactate stays at or below 2.0 mmol/L, and training there selectively loads the type I slow twitch fibers, which is what drives mitochondrial density and efficiency (remember, the powerhouse of the cell).
- Fuel use shifts with intensity, and the way he describes it, once the effort creeps into zone 3 the rising lactate actively blocks the CPT-1 and CPT-2 transporters and shuts fat oxidation down, so a harder version of the same run isn't a bigger dose of the same stimulus, it's a different one.
- Zone 2 also trains the body to clear lactate quickly, which is what lets you hold a hard effort longer later, so the easy work is what eventually pays for the hard work.
He frames all of it as preventative medicine as much as performance, insulin sensitivity and metabolic flexibility and age related decline, which is not what I came looking for but is hard to ignore.
Worth noting that the definition is a blood measurement and I am tracking all of this with a Whoop for now, which is its own problem and probably its own post.
The humbling part
Then came the humbling part. If I wanted to actually build the aerobic side of this I had to stay under that ceiling, and staying under it meant I could not run continuously, so I ended up run walking on a treadmill, which is entirely new for me. My whole mentality with running has been to push through whatever the run is doing to me, and that works well enough when the goal is to finish a distance, but it doesn't seem to work at all when the goal is aerobic adaptation, because pushing harder moves the effort out of the range that produces the adaptation I am after. The walking intervals felt like failure for about the first mile and then they just felt like the work.
What surprised me most is the gap between what I can do and what I have built. I could run for miles and still have a barely developed aerobic base underneath it. Those two things had always felt like the same thing to me and they are clearly not.
After two runs of this I can now jog the whole way at a slow pace without the walk breaks, which happened faster than I expected. Two runs is not evidence. It could be mitochondria, it could just as easily be that I have finally stopped starting every run at a pace I can't hold, or that a treadmill removes the hills and the pacing decisions and makes the whole thing artificially easy to control.
The argument on the other side
And then, because I kept watching, I ran into the argument on the other side. A GCN video made the practical case rather than the physiological one, and the numbers there are what complicate this for me. Elite cyclists ride 20 to 30 hours a week, and at that volume spending roughly 80% of the time in zone 2 isn't really a choice, it is the only way to accumulate that many hours without burning out. But for a time crunched amateur training under 6 hours a week, the argument goes, riding strictly in zone 2 doesn't provide enough overload to move VO2 max or FTP much at all, and zone 3 and 4 intervals return more performance per hour spent. Which is not a contradiction of San Millán exactly, they are answering different questions, one is describing what the tissue does and the other is describing what to do with a Tuesday evening.
My plan runs 12 to 15 hours a week, which puts me above the low volume bucket where the intensity argument is strongest and well below the elite volume where the 80/20 split is forced. I can't tell which side of that line I'm actually on. I don't think it changes what I should be doing right now, because my problem isn't that my zone 2 is too easy, it's that until very recently I didn't have a zone 2 at all, and running every mile at threshold by accident is not the same thing as doing prescribed intervals on purpose. So the base comes first either way. But somewhere in the next few months there is a question about how much intensity goes back in and when, and I don't have a way to answer it yet beyond watching what happens.
The thing I am actually watching is whether the pace at which I can hold zone 2 keeps dropping. That number moving is what would tell me the base is being built rather than the effort just being managed better, and it's the one measurement in all of this that I can take myself, every run, without buying anything.
Next
Hydration — separating the myths from the science.
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