Run the Numbers

Why I kept blowing up at interval 7

22.09.2026 · dictated by me in Italian, edited into English with Claude

My coach kept telling me I went out too hard. I only listened once an AI read my Garmin races and put numbers on my two lactate thresholds.

19 May, 12 x 800 mAverage heart rate per rep against my back-off line. Top: what I ran. Bottom: the same session called at rep 2, estimated from that day's heart rate, not a run.
19 May, what I ran and what a coach would have called Average heart rate per rep, 12 x 800 m at 10K pace minus 10 s on 19 May 2026, against a back-off line at 178 bpm and LT2 at 180. What I ran: reps 1 and 2 were the ramp-up, at 4:48 and 4:43, yet rep 2 already averaged 179: the sign of a bad day. Reps 3 to 6 went on at 181 to 183, then a long stop, and rep 7 broke at 5:28 per km; reps 10 to 12 went over the line again. Coached, estimated from that day's heart rate: called at rep 2, rep 3 at LT1 pace 5:35 and about 172 bpm, then reps 4 to 12 at 5:10 per km, all at or under 178. 19 May, what I ran LT2 180 back off 178 ramp-up Rep 1: 4:48 /km, 173 bpm 1 4:48 Rep 2: 4:43 /km, 179 bpm 2 4:43 Rep 3: 4:44 /km, 181 bpm 3 4:44 Rep 4: 4:36 /km, 183 bpm 4 4:36 Rep 5: 5:04 /km, 181 bpm 5 5:04 Rep 6: 4:48 /km, 182 bpm 6 4:48 Rep 7: 5:28 /km, 174 bpm 7 5:28 Rep 8: 5:22 /km, 175 bpm 8 5:22 Rep 9: 5:15 /km, 175 bpm 9 5:15 Rep 10: 5:01 /km, 181 bpm 10 5:01 Rep 11: 4:49 /km, 181 bpm 11 4:49 Rep 12: 4:45 /km, 183 bpm 12 4:45 the sign long stop Coached: called at rep 2, one rep at LT1, pick up LT2 180 back off 178 Rep 1: 4:48 /km, 173 bpm 1 4:48 Rep 2: 4:43 /km, 179 bpm 2 4:43 Rep 3: 5:35 /km, 172 bpm (estimate) 3 5:35 Rep 4: 5:10 /km, 177 bpm (estimate) 4 5:10 Rep 5: 5:10 /km, 177 bpm (estimate) 5 5:10 Rep 6: 5:10 /km, 177 bpm (estimate) 6 5:10 Rep 7: 5:10 /km, 177 bpm (estimate) 7 5:10 Rep 8: 5:10 /km, 178 bpm (estimate) 8 5:10 Rep 9: 5:10 /km, 178 bpm (estimate) 9 5:10 Rep 10: 5:10 /km, 178 bpm (estimate) 10 5:10 Rep 11: 5:10 /km, 178 bpm (estimate) 11 5:10 Rep 12: 5:10 /km, 178 bpm (estimate) 12 5:10 call it LT1
19 May, what I ran and what a coach would have called Average heart rate per rep, 12 x 800 m at 10K pace minus 10 s on 19 May 2026, against a back-off line at 178 bpm and LT2 at 180. What I ran: reps 1 and 2 were the ramp-up, at 4:48 and 4:43, yet rep 2 already averaged 179: the sign of a bad day. Reps 3 to 6 went on at 181 to 183, then a long stop, and rep 7 broke at 5:28 per km; reps 10 to 12 went over the line again. Coached, estimated from that day's heart rate: called at rep 2, rep 3 at LT1 pace 5:35 and about 172 bpm, then reps 4 to 12 at 5:10 per km, all at or under 178. 19 May, what I ran LT2 180 back off 178 ramp-up Rep 1: 4:48 /km, 173 bpm 1 Rep 2: 4:43 /km, 179 bpm 2 Rep 3: 4:44 /km, 181 bpm 3 Rep 4: 4:36 /km, 183 bpm 4 Rep 5: 5:04 /km, 181 bpm 5 Rep 6: 4:48 /km, 182 bpm 6 Rep 7: 5:28 /km, 174 bpm 7 Rep 8: 5:22 /km, 175 bpm 8 Rep 9: 5:15 /km, 175 bpm 9 Rep 10: 5:01 /km, 181 bpm 10 Rep 11: 4:49 /km, 181 bpm 11 Rep 12: 4:45 /km, 183 bpm 12 the sign long stop 5:28 Coached: called at rep 2 LT2 180 back off 178 Rep 1: 4:48 /km, 173 bpm 1 Rep 2: 4:43 /km, 179 bpm 2 Rep 3: 5:35 /km, 172 bpm (estimate) 3 Rep 4: 5:10 /km, 177 bpm (estimate) 4 Rep 5: 5:10 /km, 177 bpm (estimate) 5 Rep 6: 5:10 /km, 177 bpm (estimate) 6 Rep 7: 5:10 /km, 177 bpm (estimate) 7 Rep 8: 5:10 /km, 178 bpm (estimate) 8 Rep 9: 5:10 /km, 178 bpm (estimate) 9 Rep 10: 5:10 /km, 178 bpm (estimate) 10 Rep 11: 5:10 /km, 178 bpm (estimate) 11 Rep 12: 5:10 /km, 178 bpm (estimate) 12 call it LT1

“You’re going too fast.”

I’ve heard that from my coach more times than I can count, and for a long time I nodded without listening. I’d been running for two or three years and thought I knew how to train. There was some arrogance in that.

Intervals in a group pull you along. You see people ahead pushing, and a few reps in, already tired, you push anyway. Then you reach interval 7 of 12 and something gives. “Right, guys, I’m done for today.” On your way out the coach calls over: “Finish the workout. Go slower, but finish it.” So, out of pride, you finish it, with nothing left, and you pay for it the next day.

Eventually I had to admit my coach was right. I wasn’t short of effort. I was putting too much of it in the wrong places. And I couldn’t have told you what heart rate or pace “threshold” or “zone 2” meant for me. So I gave my numbers to Claude.

The two numbers

How lactate climbsFlat when you run easy, steep past LT2, and training moves both to faster paces. The typical shape, not my lab data.
How lactate climbs The typical lactate curve, not measured data: flat at easy speeds, rising from LT1 (mine 5:30 to 5:40 per km, 152 bpm), climbing steeply after LT2, classically near 4 mmol/L (mine 4:20 to 4:30 per km, 180 bpm). A dashed curve shows the same shape after training, shifted to faster speeds. 2468 lactate, mmol/L Now After training easyrunning speed →all out Easy, zone 2Threshold workAbove LT2 LT1 Lactate starts to rise Mine: 5:30-5:40/km, 152 bpm LT2 More lactate than you can clear Mine: 4:20-4:30/km, 180 bpm
How lactate climbs The typical lactate curve, not measured data: flat at easy speeds, rising from LT1 (mine 5:30 to 5:40 per km, 152 bpm), climbing steeply after LT2, classically near 4 mmol/L (mine 4:20 to 4:30 per km, 180 bpm). A dashed curve shows the same shape after training, shifted to faster speeds. 2468 mmol/L Now After training easyspeed →all out EasyThresholdAbove LT1 Starts to rise Mine: 5:30-5:40/km 152 bpm LT2 More than you can clear Mine: 4:20-4:30/km 180 bpm

LT1 is the pace or heart rate where the lactate in your blood starts to rise. Below it you’re running easy. Zone 2 lives there, and so should most of your running.

LT2 is the highest intensity where your body still clears lactate as fast as it makes it. Above it, lactate and fatigue pile up fast: you have minutes left, not an hour. When a coach says “threshold”, this is the line.

Train well and both lines move to faster paces.

What the AI gave me

It worked from two runs: my last 10K race and a 5K time trial. The number that mattered most was LT2 at 180 bpm, about 4:20-4:30 per km. The rest is in the chart. These are estimates from watch data, not a lab test, but for the first time I had numbers to aim at.

Where my paces sitPer km, easy to all out, worked out by AI from my 10K and 5K.
Where my paces sit My running paces on one line from 7:00 to 4:00 per km. Zone 2: 6:30 to 6:50. LT1: 5:30 to 5:40 at 152 bpm. Predicted half marathon: 4:38, 1:37 to 1:38. LT2: 4:20 to 4:30 at 180 bpm. The runs it used: a 10K race at 4:26 and 180 bpm, a 5K time trial at 4:13 and 181 bpm. AI estimates from Garmin data. LT1 at 152 bpm 5:30-5:40 LT2 at 180 bpm 4:20-4:30 7:00/km 4:00/km Zone 2: 6:30-6:50 /km LT1: 5:30-5:40 /km at 152 bpm LT2: 4:20-4:30 /km at 180 bpm Half marathon, predicted: 4:38 /km 6:30-6:50 Zone 2, easy runs 4:38 Half marathon pace predicted 1:37-1:38 The runs it used 10K race: 4:26 /km, 180 bpm 10K race 4:26, 180 bpm 5K time trial: 4:13 /km, 181 bpm 5K time trial 4:13, 181 bpm
Where my paces sit My running paces on one line from 7:00 to 4:00 per km. Zone 2: 6:30 to 6:50. LT1: 5:30 to 5:40 at 152 bpm. Predicted half marathon: 4:38, 1:37 to 1:38. LT2: 4:20 to 4:30 at 180 bpm. The runs it used: a 10K race at 4:26 and 180 bpm, a 5K time trial at 4:13 and 181 bpm. AI estimates from Garmin data. 7:00/km 4:00/km HM Zone 2 LT1 LT2 Z2 LT1 LT2 Zone 26:30-6:50easy runs LT15:30-5:40152 bpm Half marathon4:38predicted 1:37-1:38 LT24:20-4:30180 bpm The runs it used 10K race 4:26, 180 bpm 5K time trial 4:13, 181 bpm

19 May, looked at again

Then I pulled an old session off my watch: 19 May, 12 x 800 m at 10K pace minus 10 seconds, so 4:16 per km, with a minute’s rest. For a session that long you ramp up, and I did: the first two reps at 4:48 and 4:43. What I missed is that the ramp-up was already the answer. Reps that easy should sit well under LT2, and by the second one my heart rate was at 179, where my 10K sits. Rep 7 broke at 5:28.

It was a pace workout, but my heart rate was telling me it was a bad day, and I didn’t listen. A coach would have called it at rep 2: forget 4:16, run the rest by heart rate. One rep at LT1, 5:30-5:40, to bring it down, then pick up to whatever keeps each rep under 178. That day, that was about 5:10: at 4:40 I was 8 beats higher than the week before.

What changed

I tried the numbers first in a fartlek, and the target paces were faster than I expected. You have to trust them and settle in.

Then, on Tuesday 15 September, I ran 2-minute reps at 4:40-4:48 per km with my heart rate at 171-177, touching 180 in the shorter pieces between them. I wasn’t afraid of it, because I knew my LT2 was 180. One beat over and the session falls apart. Just under it is hard, but I can hold it.

That’s the real gain. Your coach says “threshold” or “easy”, and now it’s a number on your watch, not a guess.

It was a better session. Not because I pushed harder, but because I used exactly what my body had that day. Nothing held back, and none of the extra that used to finish me at interval 7.

Do this

  1. Go to Garmin Connect on the web and download a recent 5K or 10K. It has to be recent: give the AI old data and it describes a runner you no longer are.
  2. Give it to an AI with this prompt:
Take a look at this 5K or 10K run.
Work out my LT1, my LT2 and my zone 2.
  1. No recent race? Sign up for a 10K, or run one all out on your own.
  2. Then turn every session your coach gives you into a heart rate or a pace on your watch, with a back-off line a couple of beats under LT2.
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Run the NumbersCarmine leads a team of data analysts and races HYROX in Zurich. He trained on feel for years and got it wrong. Now he measures.
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