Usually because the zone is wrong, not because you are unfit. Watches derive zone 2 from the 220-minus-age formula, whose standard deviation is roughly 10–12 beats per minute. If your true maximum heart rate is higher than the formula assumes, every zone built on it sits too low — and the zone 2 ceiling can end up slow enough to force walking. Check it against a talk test before accepting it.
Your watch almost certainly did this: took your age, subtracted it from 220 to estimate maximum heart rate, then took 60–70% (or 60–75%, depending on the model) of that as zone 2.
Every step of that is an estimate stacked on an estimate. The formula itself was published by Haskell and Fox in 1970 and was never derived from original research — it was a line fitted through roughly a dozen existing datasets. Its authors did not present it as precise, and it has a standard deviation of about 10–12 bpm. Two 40-year-olds can have genuine maximum heart rates more than 20 bpm apart and be handed identical zones.
A 20 bpm error at the top propagates down. If the formula says your max is 180 when it is really 195, your true 70% ceiling is about 137 rather than 126 — an eleven-beat difference, which for many runners is precisely the difference between an easy jog and a walk.
There is a simple diagnostic, and it does not need equipment.
Run at an effort where you can speak a full sentence out loud without gasping — genuinely comfortable, the pace you could hold while chatting with someone. Then look at your watch.
This works because the effort at which continuous speech breaks down corresponds closely to the first ventilatory threshold — the physiological boundary that "zone 2" is trying to name in the first place. It is a measurement of you, not of a population average.
For genuine beginners, walk-run intervals during early aerobic training are normal, appropriate, and not a sign that anything is broken. Heart rate at a given pace also drifts upward with heat, dehydration, caffeine, poor sleep, illness and accumulated fatigue — so an unusually high reading on one particular day may say more about the day than about your zones.
The distinction that matters: if you can already run continuously at a conversational effort but a number on your wrist says you may not, the number is the thing to question.
A fuller comparison of the four ways to find your zone 2 →
A note on training. This is general training information, not medical advice. If you are new to exercise, returning from illness or injury, or have a heart condition, talk to your doctor before starting a training programme.
RunEasy is an Apple Watch app that runs the talk test for you. A guided ramp test steps the effort up in stages, asks a short question out loud near the end of each stage, and analyses your breathing and speech to find where it breaks down. It reports the heart-rate band that corresponds to your threshold, beside what the formula would have predicted.