08/09/2026
Garmin predicted 2:58:03. I ran 3:01:51.
Just 3 minutes and 48 seconds across a marathon. Close enough that the prediction clearly wasn’t nonsense. Far enough that it’s worth asking why.
The engine was there. VO₂max sat around 58 to 59, and two tune up half marathons in the buildup (Copenhagen 1:25:15, Chelmsford 1:24:35) backed that up. I only found out I had a London place on 9th March, so the whole build got compressed into a short, sharp block: 438km across March and April alone.
Two things stand out when I actually looked at the data properly.
First, my watch lost GPS around Canary Wharf, a known dead zone on the course where tall buildings block signal. Common issue, not a personal failing, but worth knowing your watch data isn’t gospel through that stretch.
Second, and this one’s real: my actual laps show a genuine fade from 28km to 38km, averaging 4:23/km and peaking at 4:37/km around the 30km mark. That’s not a GPS glitch. That’s durability breaking down exactly where the research says it typically does.
Which is the actual point. A recent review in IJSPP makes a case that VO₂max and even your fractional utilization of it are weak predictors of race day performance on their own. What the emerging research increasingly points to instead is durability: your ability to hold your numbers under fatigue. My splits are a pretty clean example of exactly that breaking down after 28km, while the “engine” underneath stayed the same.
A prediction is not a performance. VO₂max tells you about the engine. Training builds the capacity. But you still have to convert that fitness into 42.2km of sustained output, and that’s where pacing, fuelling, fatigue and ex*****on all get their say.
While my fitness was improving through this block, some of my recovery markers were moving the other way. HRV down, resting HR up, training readiness down. That’s what I’ll unpack next.
Data: personal Garmin export, London Marathon, 26 April 2026. VO₂max and pace predictions are Garmin’s proprietary algorithm, not lab tested.
Reference: Sondermann et al, “Fractional Utilization of VO₂max, A Key Performance Determinant in Endurance Sports?”, IJSPP 2026.
