The hype: the intensity that would fix your mitochondria
In recent years, zone 2 (that comfortable pace where you can talk without gasping) stopped being cycling coach jargon and became a longevity podcast fever. The promise was seductive: pedaling or running slowly for hours would be the optimal intensity to stimulate the mitochondria, increase fat oxidation and, as a bonus, buy years of metabolic health.
The argument gained weight when physiologist Iñigo San Millán revealed that Tadej Pogačar, Tour de France champion, spends about 80% of his training days in zone 2. If it works for the best cyclist in the world, it must be the holy grail for you too. Right? That is where the story starts to fall apart.
The counter-argument: the 2025 science pulls the rug
In 2025, researchers at Queen's and McMaster universities published a narrative review with a title that leaves no doubt about the tone: Much Ado About Zone 2. They combed through the available studies and concluded, in no uncertain terms, that the evidence does not support zone 2 as the optimal intensity for improving mitochondrial capacity, fat oxidation or cardiorespiratory fitness in the general population.
The review goes further and states the practical recommendation: prioritizing intensities above zone 2 is critical to maximize cardiometabolic gains, especially when training time is limited. In other words, the central pillar of the hype (the idea that zone 2 has unique mitochondrial power) simply does not match the data.
So is zone 2 useless? No. It works for another reason
Here comes the honest angle, and it is less glamorous than the marketing. Zone 2 is not superior for the mitochondria. It is valuable for a far more practical reason: it allows accumulating volume without breaking the body.
Easy training has lower injury risk, generates less residual fatigue, is sustainable for hours and leaves you recovered enough to absorb the hard workouts, which are the ones that actually push fitness forward. This load management role is well established in endurance science and in coaches' practice. The caveat is that it matters more for those who have already maxed out the volume of high intensity they can recover from. In practice, the elite athlete, not the recreational one with a tight schedule.
The classic mistake: copying the elite polarized model
Pogačar does 80% of his days in zone 2. But look at the denominator nobody mentions: in base blocks he trains around 25 hours per week. Elite endurance athletes live in the range of 20 to 30 weekly hours. The typical recreational athlete does 3 to 6 hours.
The polarized model (lots of easy volume, a pinch of high intensity, almost no threshold) exists to manage the elite's gigantic load. When the recreational athlete copies that 80/20 and turns it into almost 100% zone 2, they leave adaptation on the table. The intense stimulus that moves VO2max is missing, precisely the scarcest component in the routine of someone with little time.
And the intensity distribution literature reinforces this: the polarized and pyramidal models prove clearly more effective in elite and high-level athletes. In lower-level athletes, the difference between models practically disappears. Translating: for the amateur, what matters most is not the formula of the distribution, it is ensuring there is a real dose of quality in the week.
What the best evidence points to
A classic controlled study illustrates the value of well-dosed intensity: in well-trained athletes, polarized training generated an 11.7% increase in peak VO2 and 17.4% in time to exhaustion, beating models of pure volume or pure threshold.
Putting the pieces together: zone 2 is a load management tool, not a magic VO2max pill. It is the foundation that sustains volume and recovery. But a foundation alone is not a house. Without the peaks of quality (intervals, threshold, sprints), you build a wide, low base that is hard to turn into performance.
How this calibrates your week at Darwin
That is why Darwin does not build your week as 100% easy mileage or 100% intervals. The logic the assistant applies is the same the science points to: an aerobic base broad enough to sustain the load, with quality peaks calibrated by your sport (running, cycling or triathlon) and, above all, by your actual volume.
Someone training 4 hours per week does not receive the same distribution as someone training 12. The amateur with little time gets a larger proportion of quality, because that is where the gain they would otherwise leave behind lives. The high-volume athlete gets more base, because their bottleneck is recovery, not stimulus. Intensity distribution is the result, not the religion. You operate the machine; the assistant adjusts the dose to your reality, not to the routine of a Tour champion.
