Endurance
They are not competing answers to one question. They are answers to two different questions, and most people are only asking one of them.
Both, in roughly an 80/20 split. Zone 2 builds the aerobic base — mitochondria, capillaries, stroke volume — and is cheap to recover from. HIIT raises VO₂ max and top-end speed, and is expensive. Doing only HIIT plateaus and accumulates fatigue; doing only Zone 2 leaves speed on the table.
| Zone 2 | HIIT | |
|---|---|---|
| Intensity | 60–70% max HR | 85–95%+ |
| Session length | 45–90 min | 20–35 min including warm-up |
| Main adaptations | Mitochondria, capillaries, stroke volume, fat oxidation | VO₂ max, lactate tolerance, speed |
| Recovery cost | Low; repeatable daily | High; 48 h between sessions |
| Time efficiency | Poor per minute | Excellent per minute |
| Sustainable volume | Very high | Strictly limited |
HIIT genuinely delivers more cardiovascular adaptation per minute spent. If you have three hours a week and no more, intervals will get you fitter than the same three hours easy.
The limit is that you cannot simply scale it. Two to three hard sessions a week is about the ceiling for most people, because the recovery cost is real and it competes with everything else you do — including lifting. Beyond that, HIIT stops producing adaptation and starts producing fatigue, and the symptoms look exactly like being tired all the time.
| Days available | Zone 2 | HIIT |
|---|---|---|
| 2 | 1 session, 45–60 min | 1 session |
| 3 | 2 sessions | 1 session |
| 4 | 3 sessions | 1 session |
| 5+ | 4 sessions | 1–2 sessions |
Note that the number of hard sessions barely rises. Added volume goes into easy work, because that is the part you can actually absorb. This is the polarised model, and it is what elite endurance athletes have converged on across every sport.
Prioritise Zone 2 and keep hard intervals to one session a week. Hard intervals and heavy lower-body work draw on the same recovery, and the interference effect is worst when both are hard and frequent. Cycling or rowing for the Zone 2 work reduces the eccentric load further.
Neither alone. Zone 2 builds the aerobic base with a low recovery cost, while HIIT raises VO2 max and speed at a high recovery cost. The evidence-supported approach is roughly 80 percent easy and 20 percent hard.
For a limited time. HIIT delivers more adaptation per minute, but two to three hard sessions a week is the practical ceiling, and progress typically plateaus around eight to twelve weeks without an aerobic base underneath it.
One to two for most people, and one if you also lift heavily. The number of hard sessions should stay roughly constant as your training volume rises; extra volume belongs in easy work.
More than easy aerobic work does. Hard intervals and heavy lower-body sessions compete for the same recovery capacity, so hybrid athletes generally do better keeping hard running to one session a week and using cycling or rowing for easy volume.
The 80/20 split is easy to say and hard to verify, because the drift is gradual. NutriMacro records every run with pace and heart rate alongside your lifting, so you can see what proportion of last month was genuinely easy.
NutriMacro is a nutrition and training tracker for iPhone and Android. It logs food by barcode, photo or search, counts food in slices and units rather than grams, and keeps strength training, GPS activities, sleep and recovery in the same daily view. It runs in the language of your phone across 108 countries.
General training guidance for healthy adults. If you have an injury, a health condition or are new to training, get coaching before adding load or volume.
Read next: Zone 2 training explained What is hybrid training? Improving your 5K time
