On May 16, 2026, at 4:48 AM Taiwan time, the streetlights at an intersection had not yet gone dark, and this runner was already on the move.
21.22 kilometers and 1 hour 47 minutes 38 seconds later, he had a new half-marathon personal best.
This was no accident. Before this pre-dawn half marathon, he had already strung together 12 straight weeks of systematic training on Paceriz's AI plan — heading out 5+ times a week, pacing precisely by heart-rate zones every time, and steadily improving on every Tempo run.
This article fully reconstructs that journey of progress, from a starting PB of 1:50:00 to a new record of 1:46:49. All the data comes straight from his Paceriz training records.
This runner records training with an Apple Watch, connected to Apple Health and Paceriz. His manually logged starting half-marathon PB was 1:50:00 (pace 5:13/km), after which he began following the "Half Marathon 145 · 12-Week Plan" generated by the Paceriz system.
How was the 12-week plan designed?
Based on the runner's resting heart rate (49 bpm), maximum heart rate (177 bpm), starting PB, and target finish time, Paceriz automatically generated a tailored 12-week plan.
The "145" in the plan's name is the core heart-rate anchor of the training — using roughly 145 bpm as the ceiling for the low-intensity aerobic zone, so the runner can steadily build an aerobic base without accumulating excessive fatigue.
Low-Intensity Aerobic Run (Easy Run)
3–4 times a week, pace around 5:15–5:35/km, heart rate held at 125–140 bpm. Builds the aerobic base, speeds recovery, and keeps fatigue from piling up.
Tempo / Threshold Run
Once a week, 8–12 km, heart rate 141–148 bpm, pace 4:45–5:05/km. Trains the lactate threshold and improves the ability to sustain output at higher paces.
Long Slow Distance (LSD)
Once every 2–3 weeks, progressing from 15 km up to 21 km. Low heart rate (130–138 bpm) and slow pace, training fat-burning efficiency and mental endurance.
Intervals / Progression Run
Added occasionally to stimulate VO2 max and the neuromuscular system, maintain a sense of speed, and keep the body from over-adapting to a single intensity.
22 weeks of stacking training volume
From his first data sync in December 2025 to the day of the half marathon in May 2026, this runner completed a total of 135 running sessions, with total mileage over 1,000 kilometers.
Plan 37.4km, 126% completed
The overall intensity distribution is healthy: about 65% low intensity, 20% medium, and 5–10% high. This closely matches the "80/20 training" used by elite marathon runners — building the aerobic engine with plenty of slow running and stimulating the cardiorespiratory ceiling with a small amount of high intensity.
The four-phase design of the Paceriz plan
These 12 weeks were not a straight line, but a cycle of four phases joined together with precision:
Aerobic base phase Hold down the high-intensity share and push up low-intensity volume, letting the cardiorespiratory system build an aerobic engine without excessive fatigue W5 high intensity was only 1.1%, yet weekly mileage reached 52.7km; marathon-zone pace 5:03→4:57/km
Volume-peak phase W6 deliberately drops back (35.4km) so the body absorbs the earlier training, then W7+W8 push to the highest volume of the whole cycle, triggering aerobic supercompensation W7 54.8km + W8 47.4km totaling 102.2km, 5–6 weeks out from race day — exactly the aerobic adaptation window
Intensity-shift phase With the aerobic base built, "shift gears" and add speed stimulus so the body starts getting used to race pace High-intensity minutes: W9 8 min → W10 19 min → W11 29 min (increasing week over week)
Taper activation phase Total volume contracts 34% to let muscle glycogen recover, while high-intensity density hits its cycle peak to keep the neuromuscular system activated, arriving at the start line in peak condition High intensity 50 minutes (7× that of W1), total training volume down to 186 minutes (vs 281 minutes in W9)
The marathon zone showed a pace of 5:03/km in both W1 and W8 — but W8's heart rate was 132.2 bpm, 2.5 beats lower than W1's 134.7 bpm. Same speed, an easier heart: this is the most direct physiological marker of a deepening aerobic base, and the reason he could sustain steady output all the way to the finish on race day.
Pace progress on mid-distance fast runs
Below are the actual recorded paces from several representative mid-distance fast runs (8–12 km), showing how speed evolved across the training cycle:
From 4:57/km in December to a fastest 4:46/km in April, that's about an 11 sec/km improvement. The slight pace pullback after entering the taper in May was a deliberate easing of intensity to let the body recover, not a regression.
The mileage evolution of LSD runs
Long slow distance (LSD) is the backbone of half-marathon training. The Paceriz plan progressed LSD distance from 15 km up to the full half-marathon distance, so the runner felt completely at home with the distance before race day.
Race day: steady output at 5:00/km
4:48 AM, somewhere in Taipei, the streetlights still dim. This half marathon had no official starting gun — the target race date set in Paceriz was the next day, May 17, but he chose to go a day early, with only his own self-set starting point.
His average pace for the full distance was 5:04/km, dynamic VDOT 44.87 — about 4 units higher than the VDOT corresponding to his starting registered PB (1:50:00). This was a clear "race pace" rather than an easy jog, meaning the runner was genuinely pushing his own limits.
(Feb 20, training run) Latest half
(May 16, new PB)
Recalculating this runner's pre-race state with the current algorithm:
5/9 (one week out): predicted 1:40:56, VDOT 45.7, good recovery (85 points)
5/15 (one day out): predicted 1:42:16, VDOT 45.4, poor recovery (62 points), HRV still declining
5/16 actual result: 1:46:49
Prediction and actual differed by about 4.5 minutes. The source of the gap is clear: the day before the race, the system had already issued a warning — "insufficient recovery, suggest resting 2–3 days" — but this runner chose to ignore it and set off with a fatigued body. The fitness was there; the recovery was not.
How far from the goal?
The goal Paceriz set for this runner was 1:45:00 (4:58/km), with the target race date set for May 17. This half marathon took place on May 16 — the day before the target race day he had set in Paceriz, completing this showdown a day early. The 1:46:49 he ran was just 1 minute 49 seconds off the goal.
(target date: 5/17)
(5/16, one day early)
(achieved the day before target date)
In other words, the day before his self-set target race date (May 17) arrived, this runner's form had already reached 97.9% of the goal.
4 key changes the AI plan delivered
Slow first, fast later: just 1.1% high intensity in the first 5 weeks
W5 covered 52.7km but only 3 minutes (1.1%) were high intensity. The plan deliberately held down intensity to first build up the aerobic engine's displacement — that's the fundamental reason he could later sustain a steady 5:04/km in the race.
Volume peak: W7+W8 totaling 102.2km
The back-to-back peaks in weeks 7 and 8 added up to 102.2km, exactly 5–6 weeks out from race day — the golden window for aerobic supercompensation. Not random ambition, but precisely calculated timing from the plan.
Same speed, lower heart rate = improved aerobic efficiency
In training week 8 the marathon-zone pace was 5:03/km, the same as week 1, but heart rate dropped from 134.7 to 132.2 bpm. The heart working more efficiently at the same speed is the key to not fading in the later stages of the race.
Taper isn't rest: high-intensity density hits a new high
In the final week, total volume shrank 34%, but high-intensity minutes reached their cycle peak (50 min), 7× that of week 1. Full glycogen plus a neuromuscular system kept activated is what makes the explosive performance possible on race day.
"The steady output on race day doesn't come from good form that day — it comes from the accumulation of executing every week's plan well."
— Core of the Paceriz training philosophyFrom 1:50 to 1:46: let the data speak
Putting all the numbers together, this training journey can be summed up like this:
This 4:48 AM half marathon wasn't run on a stroke of good form. 135 training sessions, 558km accumulated over the 12-week plan, and a cycle design with four precisely connected phases — that's the real reason behind these 3 minutes 11 seconds of progress.
On February 20, he ran the half-marathon distance for the first time, in 5:30/km. On race day he covered the same distance at 5:04/km — those 26 seconds were earned by the 558km the Paceriz plan guided him through.
Train with a basis, and progress is solid.