It felt like 17°F at the trailhead.
I had two choices: hold a steady pace to stay warm (and let my heart rate spike on the hills), or slow down to preserve perfect Zone 2 distribution.
I chose warmth over perfection.
The woods were empty. Just me, the cold, and a 5K loop I’ve run a hundred times. The goal was simple: steady effort, keep moving, stay functional.
Not a workout. A session.
The Decision
Every coach knows the Zone 2 doctrine. Easy runs stay easy. Conversational pace. Build the aerobic base without accumulating fatigue.
But doctrine doesn’t account for 17-degree mornings when slowing down means shuffling and dropping your core temperature. I needed to generate heat.
Context matters.
I decided to hold a steady rhythm through the entire loop. That meant deliberately hitting Zone 3 on the climbs rather than walking them to preserve my heart rate zones.
The alternative was getting cold and missing a misunderstood, often maligned, yet beneficial training stimulus.
The Data
36 minutes flat for the 5K.
Here’s what the heart rate file showed:
Zone 2: 22.5 minutes (62%)
Zone 3 + Zone 4 combined: 9.2 minutes (25.5%)
Zone 1: The rest
Not perfectly pyramidal. Close enough.
Every spike above Zone 2 came from a hill. The flats stayed solidly aerobic base—no random drift. No ego pushing. Just terrain doing what terrain does when you maintain effort instead of pace.
By the time I finished, I’d stayed warm, held good form throughout, and logged exactly what I needed: an aerobic session that built base without wrecking recovery.
The Principle
There’s a religion around Zone 2 training. It treats the heart rate monitor like a judge.
Stay conversational. Never let it creep. The moment you touch Zone 3, you’ve failed.
This works perfectly in a lab. It works when you have 60-degree weather, flat terrain, and unlimited time.
The rest of us train in the real world.
In sub-20-degree temperatures, on a short 5K, holding a steady pace matters more than holding back. Your body needs to generate heat. Your form needs a consistent rhythm. Your aerobic system still benefits when the majority of the session is Zone 2, and above that is deliberate and strategic.
The question isn’t “Did I stay in Zone 2 every second?”
The question is: “Did I get a training stimulus without compromising what matters?”
On this run, the goal was aerobic base building that maintained form and kept me functional in cold conditions. Mission accomplished.
The Threshold
Strategic Zone 3 has a clear boundary.
Under 25% of total session time? Acceptable, especially when tied to specific terrain features, such as hills.
Over 30%? You’re living in the gray zone, accumulating fatigue without triggering the adaptations that come from easy aerobic base or actual threshold work.
My 25.5% sits right at the edge. If this run had been 40 minutes with 12 minutes in Zone 3, I’d have crossed into problematic territory. But 9 minutes out of 36, all from deliberate hill climbs?
That’s context-dependent training, not zone drift due to a lack of discipline.
What This Means for You
If your training plan says “60 minutes Zone 2,” but it’s 15 degrees outside and your route has hills, don’t walk every climb to preserve your heart rate distribution.
Hold steady effort. Let the hills push you into Zone 3. Keep the total high-zone time under 25% of the session.
You’ll build the same aerobic base you would have with perfect Zone 2, you’ll maintain better form than if you’d shuffled, and you’ll actually complete the session instead of bailing because you’re freezing.
Elite athletes with 15 hours a week to train can separate every stimulus into isolated perfection. The rest of us need to train smart within the constraints we actually face.
After 25 years of coaching endurance athletes, I can tell you this: The ones who last decades aren’t the ones who follow every rule religiously.
They’re the ones who understand principles well enough to apply them contextually.
Cold morning. Steady pace. Hills spiked my heart rate.
Close enough.
Ed is a Certified Strength and Conditioning Specialist (CSCS) and UESCA Ultramarathon Coach with 25+ years of experience. He’s currently training for 50Ks and Ironman events as he approaches age 50.
