TL;DR
- Zone 2 builds your aerobic base: fat oxidation, mitochondrial density, and durability.
- HIIT (intervals) is efficient for pushing VO2 max, but it is stressful and easy to overdo.
- The best plan for most people is not either/or. It is mostly Zone 2 plus a small dose of intensity.
- If your sleep, HRV trend, or resting heart rate is off, reduce intensity before you cut easy volume.
- Century helps you balance training load with recovery so you can progress for months, not weeks.
Why this question matters
Zone 2 is popular because it is sustainable.
HIIT is popular because it is fast.
People usually ask the question when they feel stuck:
- "I run a lot but I am not getting faster"
- "I do intervals but I feel cooked"
- "My VO2 max is not moving"
The right answer depends on your current training history, your recovery capacity, and your week.
What Zone 2 actually does
Zone 2 is steady, conversational pace work.
It improves:
- aerobic efficiency
- ability to sustain work without accumulating too much lactate
- recovery between harder sessions
- long term durability (tendons, connective tissue, pacing discipline)
Zone 2 is also the training you can repeat week after week without frying your nervous system.
What HIIT actually does
HIIT includes workouts like:
- 4 x 4 minutes hard with easy rest
- 6 to 10 x 1 minute hard
- hill repeats
It improves:
- maximal oxygen uptake (VO2 max) stimulus
- speed and power at higher intensities
- your ability to tolerate discomfort
But HIIT comes with costs:
- higher injury risk if you build too fast
- bigger recovery demand
- easier to stack too much intensity across the week
The common mistake: too much "medium hard"
Many people accidentally train in a gray zone:
- not easy enough to recover
- not hard enough to get a clear VO2 max stimulus
If you are always "kind of pushing", you can stall.
A better pattern is:
- most sessions truly easy (Zone 2)
- a small number truly hard (intervals)
So which improves VO2 max more?
If you only care about VO2 max quickly, HIIT tends to create a stronger short term signal.
But long term, the people with the best VO2 max usually have:
- a big aerobic base (years of easy volume)
- consistent training without interruptions
Zone 2 is what makes consistency possible.
That is why the best answer for most people is:
- build Zone 2 volume
- add 1 to 2 intensity sessions per week (depending on training age)
A simple weekly split (most people)
If you train 4 to 6 days per week:
- 2 to 4 days Zone 2
- 1 day intervals (HIIT)
- 1 day long easy session (still Zone 2)
- optional: 1 day strength training
If you train only 3 days per week:
- 2 days Zone 2
- 1 day intervals
If you are new or returning:
- start with all easy training for 3 to 6 weeks
- then add a light interval session
How to know you are overdoing HIIT
Signs you are stacking too much intensity:
- sleep quality drops
- resting heart rate trend rises
- HRV trend drops
- you feel flat in warm ups
- easy pace feels unusually hard
When this happens, do not add more intervals.
Reduce intensity for a week and keep easy volume.
Two useful videos on aerobic base and intensity
Where Century fits
Training advice is easy to read and hard to execute when life happens.
Century helps you stay consistent by:
- tracking your recovery signals alongside your training load
- showing when your body is absorbing the work vs accumulating fatigue
- helping you plan intensity so you do not accidentally turn every day into a hard day
If you want VO2 max progress without burnout, the key is the right dose, repeated for months.