You said:
So I am sustaining zone 4 or 5 for nearly an hour. I thought that was impossible?
If most of your runs look like this:
10 km
~165 bpm
55 minutes
You’re probably stuck in the “moderate hard” grey zone, which:
Builds fatigue
Limits aerobic base growth
Slows long-term improvement
👉 165 bpm at age 56 is not Zone 2 — it’s near threshold.
👉 True Zone 2 will feel almost frustratingly easy, especially at first.
If you want, I can:
Estimate your zones more precisely (using resting HR)
Help you translate Zone 2 into pace for Thai heat
Suggest how to balance Zone 2 with faster work without losing fitness
It sounds impossible — but what you’re experiencing is very common, and it comes down to how zones are defined vs how the body actually works.
Short answer:
👉 You are not sustaining true Zone 5 for an hour.
👉 You are very likely sustaining upper Zone 3 / Zone 4 (near lactate threshold) for ~55 minutes.
Let’s clear the confusion.
True Zone 5 (VO₂max):
~90–100% of VO₂max
HR near absolute max
Sustainable for 3–8 minutes, maybe 10–12 minutes if very trained
So yes — holding actual Zone 5 for an hour is physiologically impossible.
The issue is HR-based zone models, especially:
Using age-predicted max HR
Using generic tracker zones
For some people:
Their true max HR is higher than predicted
Their lactate threshold occurs at a very high % of max HR
Endurance-trained runners often race at:
85–92% of true max HR for ~1 hour
That’s exactly what your data looks like.
A 55-minute 10 km at 165 bpm suggests:
You’re running around lactate threshold
This is typically called:
Zone 4, or
“Tempo / threshold” intensity
Threshold effort can be sustained for:
40–70 minutes in trained runners
So your performance is normal and realistic.
At 25°C:
HR can drift +5–15 bpm
Cardiovascular strain increases
Actual metabolic intensity may be lower than HR suggests
So:
Your metabolic zone may be threshold
Your heart rate appears closer to max
This is called cardiac drift, not overexertion.
Zone charts assume:
Fixed max HR
Fixed relationship between HR and effort
But in reality:
Zones shift with heat, fatigue, hydration, and fitness
HR lags behind actual workload
That’s why elite runners often train by:
RPE + pace, not HR alone
Ask yourself:
Could you speak more than 2–3 words at a time? → threshold
Could you accelerate hard at the end? → threshold
Did you feel close to all-out? → then maybe VO₂max (unlikely)
If you finished controlled, you were not in Zone 5.
You are:
Not doing the impossible
Running near lactate threshold for ~55 minutes
Using zones that don’t match your physiology
👉 Your heart rate number looks extreme, but your effort is not.
If you want, I can:
Estimate your true max HR
Approximate your lactate threshold HR
Show how to build a zone model that actually fits you
Just tell me whether you know your resting HR or have ever done a hard max-effort test.
Great — that helps a lot. With a resting HR of 58 bpm, we can put real numbers on this and clear up the paradox.
Let’s assume a true max HR ≈ 170 bpm
(This fits your ability to run hard at ~165 without blowing up.)
HRR = Max − Rest = 170 − 58 = 112
Now apply Karvonen (HRR) zones, which work much better for trained runners.
| Zone | %HRR | HR (bpm) | What it feels like |
|---|---|---|---|
| Zone 1 | 50–60% | 114–125 | Very easy |
| Zone 2 | 60–70% | 125–136 | Easy aerobic |
| Zone 3 | 70–80% | 136–147 | Steady |
| Zone 4 (Threshold) | 80–90% | 147–158 | Hard but controlled |
| Zone 5 | 90–100% | 158–170 | VO₂max |
Average HR: 165 bpm
That is ~95% of HRR
But only ~97% of predicted max, not true max effort
This tells us something important:
👉 Your max HR is probably higher than 170
👉 Your lactate threshold HR is unusually high (very common in fit runners)
In other words:
You are not in true Zone 5
You are sitting just above threshold, pushed upward by heat and cardiac drift
Well-trained runners can sustain:
88–92% of true max HR for ~1 hour
Especially in warm conditions where HR inflates
Your physiology likely looks like:
Lactate threshold HR ≈ 160–165 bpm
Which matches your experience perfectly