Measurement log
A new Garmin's race predictor spends its first month guessing
My Forerunner 265 said 1:54:56, then 1:36:46 nine weeks later. My actual pace barely moved. Here is what the predictor was really doing.
Six days after I first wore a Forerunner 265, it told me I would run a half marathon in 1:54:56. Nine weeks later the same watch said 1:36:46 — eighteen minutes faster.
I did not get eighteen minutes faster in nine weeks. Nobody does. So I exported the data to find out what the number was actually tracking.
The curve
Source: my own Garmin account export, RunRacePredictions, 187 entries between 23 June and 23 August 2026. Chart shows the last value recorded on each day.
Two thirds of the entire improvement — 13:03 of 18:10 — happened in the first three weeks. After that the line settles into a slow, plausible drift of about five minutes over six weeks.
A curve shaped like this is not a fitness curve. Fitness does not arrive in a rush and then stop. This is the shape of an estimator with almost no data, correcting itself as data arrives.
What my fitness was actually doing
This is the part most people cannot check, and the only reason I can is an accident of timing. The Forerunner replaced an Apple Watch Series 10 that had recorded every run since March 2025. So I have sixteen months of running from before the watch existed, and I can ask a simple question: at the same heart rate, was I running faster after the switch than before it?
| Average heart rate | Apple Watch, Jan–Jun 2026 | Forerunner 265, Jun–Aug 2026measured |
|---|---|---|
| 145–155 bpm | 5:38 /km (n=6) | 5:49 /km (n=5) |
| 155–165 bpm | 5:33 /km (n=23) | 5:16 /km (n=9) |
| 165–175 bpm | 5:11 /km (n=13) | 5:06 /km (n=5) |
At my easiest efforts I was slightly slower. In the middle band I gained about seventeen seconds per kilometre; at the top, five. Weekly volume went from 20.9 km to 25.1 km.
That is a real improvement, and I will take it. It is not eighteen minutes of half marathon. Call it three to five, generously. The remaining thirteen minutes were the watch learning who I was.
What this means if you just bought one
Three things follow, and only the first is obvious.
Ignore the first month entirely. Not “take it with a pinch of salt” — ignore it. A brand-new watch has no VO2 max history, no baseline, and no idea whether your first few runs were easy or hard. My first prediction was eighteen minutes pessimistic. Someone whose first recorded runs happen to be intervals will get the opposite error in the other direction.
A big early improvement is not evidence your training is working. This is the trap, and it is a pleasant one to fall into. Watching the number tumble week after week feels like proof. It was proof of nothing except that the watch had started with a bad guess.
Judge the predictor on its slope, not its value. Once the estimate has settled — for me, from about the third week of July — the week-to-week movement starts meaning something. Before that, there is no signal in it at all.
The honest limitations
The comparison above puts two different watches’ heart rate sensors side by side, and they do not measure identically. A systematic difference of a few beats between an Apple Watch and a Forerunner would shift the bands and change the numbers. The sample sizes in the Garmin column are small — five to nine runs per band. And my volume rose by a fifth across the switch, which is part of the genuine improvement I credited above.
None of that rescues thirteen minutes. But it is the reason I am saying “three to five minutes of real gain” rather than putting a decimal on it.
The test that settles it
All of this is still a prediction against a prediction. On 13 September 2026 I run the swb-Marathon Bremen half, and on 25 October the hella hamburg halbmarathon. The watch currently says 1:36:46. I will publish both actual times against what the watch predicted the morning of each race, whether or not it makes the watch look good.
That page will stay at the top of this site and get a new row after every race.
Sources
- My own Garmin Connect account export:
RunRacePredictions_20260617_20260823, 187 daily entries, andsummarizedActivities, 26 running activities. - My own Apple Health export, 24 August 2026: 157 Apple Watch running workouts, March 2025 to 15 June 2026.
- How to pull these yourself: Garmin Connect → Account Settings → Export Your Data; Health app → profile → Export All Health Data.