Zone 2 training is having a moment — and for good reason. Once the domain of professional cyclists and marathon runners, the research on low-intensity aerobic training has gone mainstream. Here's what it actually is, why it works, and how to use it.
What is Zone 2 training?
Zone 2 is exercising at 60–70% of your maximum heart rate. In practical terms, this means a pace where you can hold a full conversation — but wouldn't want to. You're working, but comfortably. On a scale of 1–10, it's about a 4–5 out of 10 effort.
For a 35-year-old with an estimated HR max of 185 bpm, Zone 2 runs from roughly 111 to 129 bpm. This is likely slower than you think.
Why is it so effective?
Zone 2 is where your body primarily runs on fat oxidation (burning fat as fuel), rather than glycogen (sugar). Training at this intensity repeatedly triggers the following adaptations:
- Mitochondrial biogenesis — you grow more mitochondria in your muscle cells, the "power plants" responsible for aerobic energy production
- Improved fat oxidation efficiency — your body gets better at using fat as fuel, preserving glycogen for high-intensity efforts
- Cardiovascular efficiency — stroke volume (how much blood your heart pumps per beat) increases, meaning your heart works less for the same output
- Lactate clearance — Type 1 (slow-twitch) muscle fibres develop enhanced ability to clear lactate, improving your sustainable threshold
The research behind it
Dr. Iñigo San Millán, researcher and physiologist at the University of Colorado, has published extensively on the importance of Zone 2 for both metabolic health and athletic performance. His work with Tour de France cyclists shows that elite riders spend the majority of their training time — often 75–80% — in Zone 2.
Dr. Peter Attia, one of the most prominent longevity researchers, cites Zone 2 as one of the four pillars of physical health for long-term quality of life. He recommends a minimum of 3 hours per week of Zone 2 work for health maintenance.
How much Zone 2 training should you do?
Research and elite coaching both point to 3–4 sessions per week, each lasting 45–90 minutes, as the range that produces meaningful adaptation for recreational athletes. That's roughly 3–6 hours of Zone 2 per week.
If that sounds like a lot, start with two 45-minute sessions per week and build from there. The key is consistency over months, not heroic single sessions.
The common mistake: going too hard
The most common error people make is drifting into Zone 3 during easy runs. It feels almost dishonestly slow at first — especially for runners used to training by pace. True Zone 2 requires running or cycling at a pace that feels almost embarrassingly easy.
Using a heart rate monitor (or Apple Watch) is the only reliable way to stay in Zone 2. Perceived exertion consistently overestimates intensity for trained athletes.
Zone 2 and Underdog
Underdog tracks every minute you spend in each zone. Zone 2 earns points toward your weekly league total — and because it rewards time, not pace or distance, it's the most level playing field possible. A 30-minute Zone 2 walk earns the same points as a 30-minute Zone 2 jog, because both produce the same physiological stimulus.
This makes the Underdog league system uniquely suited to Zone 2 training: you earn points whether you're a competitive runner or someone who just started.
Bottom line
Zone 2 training is the unsexy, unglamorous foundation of aerobic fitness. It's not HIIT, it's not impressive to post on social media — but it's the single most research-backed way to improve your cardiovascular health, metabolic efficiency, and long-term endurance capacity. Slow down to go fast.