Am I ready for a marathon?
My watch says 4:27. My data says I'd walk at km 18.
Your watch doesn't answer it directly. VO2max, race predictions and Endurance Score won't tell you if you'll finish. The best signal I found in my own data is Stamina, recorded on your training runs too: on your last long run, see at which kilometre its trend hits zero. Before the half marathon I finished with 14% Stamina left, that point was 24.8 km. My working rule: you're ready for a distance when that point sits about 20% beyond it.
Two numbers, one watch
Monday night, 28 September. I open the race predictor on my Forerunner 955. Half marathon: 2:00. Marathon: 4:27. Tempting, right?
Then I pull the Stamina data from my longest recent run and extend the line. It hits zero at km 18.
Same watch, same week. One number says I could finish a marathon. The other says I’d be walking before the end of a half.
So which one do I believe?
The question every plan dodges
I’ve worn a Garmin for almost ten years. Trails, the 45 km Trail de Paris, the Amsterdam marathon in 2021, and a lot of half marathons over the last two years.
And honestly? Until last year, I had very little confidence that I’d finish well. Not finish. Finish well.
Every race starts the same way. You pick a 12 or 16 week plan, and the plan makes you a promise: do this, and you’ll finish the race at that time. Except it’s random, and it’s often wrong. Your body needs a lot more than 12 weeks to take the hit of a marathon.
The question I actually want answered is simpler. With my current training, if I keep going like this, will I be able to run this race?
The line starts at the half. 21.1 km is not nothing. At Marseille-Cassis, a hugely popular race, I’ve watched runners who clearly hadn’t taken it seriously drop out along the way. And the marathon is another world: running economy, when to drink, when to eat. It’s not nothing either.
So I went looking for that answer in my own watch. Here’s what I found.
Everything I checked before a race, and why none of it answered
| What I looked at | Why it didn’t answer “will I finish?” |
|---|---|
| VO2max | It measures your engine, not how long it lasts. Not much use past 2 hours |
| Race predictions | Today my watch says 2:00 for a half and 4:27 for a marathon. Everything else in my data says no (more below) |
| Strava fitness | No benchmark. What number do you need for a marathon? Nobody knows |
| Endurance Score | Abstract. I’m at 5,656, “Intermediate”. Does that finish a marathon? No idea |
| Weekly mileage rules | Scattered advice, a different number on every site |
| Acute load | Closer, since it’s built on volume. It’s what I watch now, with how easy my long runs feel |
And there’s the long run problem. Nobody runs 42 km in training. So you run 2 hours and hope it’s enough.
That’s what’s missing most: not another metric, confidence. One calculation that says: yes, you can do this.
Stamina was in my training runs all along
For years I thought Stamina only showed up in races. So, useless for preparing: you learn you’re out of fuel at km 31, when it’s too late.
I was wrong. My Forerunner 955 records Stamina on any run, training included, and Garmin Connect keeps a Stamina chart for each activity after the run.1 The minimum it needs to work is your VO2max estimate.2
That changes everything, because on a long run you can read two things:
- How much is left at the end. A 2-hour run finishing at 50% is not the same run as one finishing at 10%.
- How fast it drains. Stamina drops almost in a straight line on a steady run. So you can extend that line and see where it would hit zero.
Extend the line, find the zero point
That zero point is my candidate for the missing number. Not “how fit am I”, but how far this body would go at this pace, today.
Nerd note. The drain is strikingly linear: on the three runs below, a straight line explains more than 99.9% of the drop (R² between 0.9994 and 0.9998). Stamina is Garmin’s model, not a lab measure, so I never read it alone. I cross it with heart rate drift, right below.
Heart rate drift, the second opinion
Stamina is Garmin’s model. Heart rate drift is plain physiology, and it’s my cross-check.
The idea is simple. On a steady run, same pace, your heart should beat at the same rate from start to finish. When the heart starts beating faster for the same pace, your body is paying more for the same work. That’s drift.
Coach Joe Friel turned it into a number called aerobic decoupling. You split a steady run in two halves. For each half, you divide your pace by your heart rate: that’s your efficiency. Then you compare the two. TrainingPeaks, which computes it as Pa:HR, reads under 5% as good aerobic endurance at that duration, and says it isn’t valid on efforts under 20 minutes.3
Your Garmin doesn’t show it. But the data is all there, second by second. So I computed it on my runs: grade-adjusted pace divided by heart rate, first half against second half, first 10 minutes left out as warm-up.
| Run | Duration | Heart rate, 1st half → 2nd half | Decoupling |
|---|---|---|---|
| Long run, May 2025, 22 km | 2h09 | 144 → 146 bpm | 0.2% |
| Lyon half, Oct 2025 | 1h44 | 164 → 168 bpm | 3.2% |
| Easy run, Aug 2026, 9 km | 57 min | 140 → 144 bpm | 3.4% |
This is the line that hurts. In May 2025, my heart held steady for over two hours. This summer, on an easy 9 km, it already drifted more than during a half marathon at race pace.
Same body, same watch. What changed is the volume behind it. And Stamina tells the exact same story: 34.7 km of range then, 18 km now. When two different signals agree like that, I start to believe them.
Nerd note. The 5% line was built for steady aerobic efforts. A half at race pace isn’t one, so read my Lyon number as a bonus, not a textbook case. Heat, stress and dehydration push drift up too, so compare runs in similar conditions.
Would it have predicted my own races?
A number is only worth something if it would have told me the truth before. So I replayed it on my past runs.
| Run | Pace | Stamina at the end | Zero point | What happened |
|---|---|---|---|---|
| Long run, May 2025, 22 km | 5:52/km | 36% | 34.7 km | Nine days after my 1:48 at the Nîmes half, in the middle of my best spring |
| Lyon half, Oct 2025, 21.2 km | 4:55/km | 14% | 24.8 km | Finished in 1:44, empty but in one piece |
| Easy run, Aug 2026, 9 km | 6:23/km | 51% | 18.1 km | Today’s shape (more below) |
My case: the Lyon half
October 2025. On paper, I was less trained than for my previous halves: around 20 km a week in the two months before, against close to 37 in the spring.
And yet I lined up confident. Not because of a number on my watch. Because my long runs felt good.
The race data backs that feeling. I crossed the line in 1:44 with 14% Stamina left, and the line would have hit zero at 24.8 km, 17% past the finish. Tight, but true.
What I don’t have is that same reading from my long runs before Lyon. “I feel good on my long runs” was my only gauge. Turning that feeling into a number you can check is the whole point of this piece.
And Amsterdam? My watch didn’t have Stamina in 2021. What I do know: about 210 km in the six weeks before, long runs of 21 to 23 km, and a wall at km 31. I’d love to know where that zero point would have been. What my watch did tell me that day was Unproductive, and I decoded that Garmin label with my own numbers.
My working rule: you’re ready for a distance when your last long run puts the zero point about 20% past it, at your race pace. Around 25 km for a half. Around 50 km for a marathon.
Two races is not proof. It’s a hypothesis, and here’s what would kill it: a half where I blow up with a zero point above 25 km.
So, am I ready for a marathon today? Nope.
I ran the numbers on myself this week, 28 September 2026. It stings a bit.
Since May I’ve averaged about 12 km of running a week, 25 at my best. My longest run is 9 km. On it, the zero point landed at 18 km.
So for a half: doable if I grit my teeth, not comfortable. That’s exactly the Marseille-Cassis scenario I described above, and I’d be the one walking.
A marathon: no.
And my watch? Still promises 2:00 and 4:27. Garmin says those predictions come from your VO2max estimate and your training history.4 In my case, the history clearly doesn’t weigh enough: I haven’t run more than 9 km in five months.
The good news is the number tells me what to do, not just where I stand. Get my long run back to 16 to 18 km, my week back towards 30 km, and watch the zero point climb past 25 km. Adding about 10% a week, that’s roughly six to eight weeks for a half. For a marathon, think months.
Still true on today’s watches?
Garmin is moving in this direction. With the fēnix 9 in August 2026, Garmin Connect added a stamina curve: an estimate of how long your stamina can hold different paces, split into five stamina zones.1
That’s close to my zero point. But it assumes you’re recovered and pacing steadily,1 and it still doesn’t tell you “ready” or “not ready” for a race distance. That last step is still on you.
Where this breaks
- One runner, two races. Mine. Your body, your watch, your pace will move the numbers.
- A straight line is a bet. Past the distance you actually ran, I’m extending a trend. Fatigue late in a marathon may not be linear at all.
- Pace matters. The zero point only holds at the pace of that run. An easy 6:23/km run says little about a race at 5:30.
- Fuel isn’t in the model. Gels, water, heat. A marathon is also a question of when you drink and when you eat, and no watch metric sees that.
That’s why I cross Stamina with heart rate drift. If my heart climbs at the same pace and Stamina melts at the same moment, I believe it. If they disagree, I don’t trust either.
Test your own case
Where to find your numbers: in Garmin Connect, open the run and look for the Stamina chart.1 No chart? Add the Stamina data screen to your running activity profile, then check again after your next run.1
Try it · 30 seconds
Am I ready?
Copy one long run from Garmin Connect, or start from one of mine.
What’s next
I’m going to replay this after every long run, on myself, in public. No race on the calendar before the end of the year. The goal is bigger: next year, much longer runs, with ease. First step, a zero point past 25 km.
Want to see if the rule holds, or if I end up walking at km 18? That’s in the next letter.
Footnotes
-
Garmin Wiki (Garmin Rumors), Real-Time Stamina, updated August 2026 ↩ ↩2 ↩3 ↩4 ↩5
-
Garmin Blog, Introducing the Garmin Real-time Stamina feature ↩
-
TrainingPeaks Help Center, Aerobic Decoupling and Efficiency Factor ↩
One metric, decoded. Every other Sunday.
Straight answers from someone who wears the watch too. No spam, unsubscribe in one click.