Pace measures what you did. Heart rate measures what it cost you — and the cost is what you're training.
Run the same loop twice at the same pace, once on a cool March night and once in August, and the second one will nearly kill you. The pace was identical. The effort was not.
That is the case for heart rate in one sentence: pace is an output, heart rate is a cost. Training works on the cost.
Where pace lies
- Hills. There is nowhere flat in Yerevan. A 6:00/km up through Kond and a 6:00/km back down are completely different runs.
- Heat. Above roughly 20°C your body starts spending blood on cooling you instead of on your legs. Same pace, higher cost.
- Tiredness, bad sleep, a cold coming on, too much coffee. All of it shows up in heart rate and none of it shows up in pace.
The zones, briefly
Most watches split your range into five. For our purposes only two matter:
- Zone 2 — easy. You can talk. This is where most of your running lives.
- Zone 4 and up — hard. You cannot talk. This is where intervals live.
Zone 3 is the trap — the not-quite-easy, not-quite-hard middle that tires you out without improving much.
Getting your zones roughly right
Your watch guesses your maximum heart rate from your age, usually as 220 minus it. That formula is wrong by about 10–12 beats either way for most people, so treat the zones as a starting point rather than as physics.
Calibrate them against the talk test. If your watch says Zone 2 but you cannot speak a full sentence, your zones are set too high — not your effort too low. Trust the sentence.
Two honest caveats
Heart rate lags. For the first five or ten minutes it is still catching up, so ignore it early in a run.
And it drifts upwards over a long run even at constant effort, especially in heat. That is normal, and it is information: your body really is working harder to hold the same pace.
If you would rather have the targets written down for you, the heart-rate version of the half marathon plan gives a range for every session.