Long run · Sunday, 1 November 2026 · Week 10, Durability Base

A long run holds together, just, as effort drifts late

21 weeks to MDS

Fifteen point three miles, the long run for week ten, done as planned despite the humidity. Average heart rate sat five under the cap for the whole session, and only nine per cent of the time went above it.

But the shape of the run tells a different story. The first mile went at 13:08 with heart rate at 115, and by the final full mile that had slipped to 13:39 with heart rate at 138. The running became 5.4 per cent less efficient late on, speed per heartbeat falling just outside the band I call held, and the two halves decoupled by 7.5 per cent, pace against heart rate drifting further apart than I'd like, starting from around two hours twenty in.

Nothing here is alarming on its own. I'll look at where in future long runs that drift starts, and whether it moves earlier or later than this.

Distance: 15.3 mi; 24.6 km
Moving time: 3h 30m; started 07:51
Average pace: 13:45; per mile
Heart rate: 126; average · max 167
Climb: 387 ft; 118 m
Conditions: 9°C; 93% humidity · wind 7 km/h

Planned long run, full distance.

Heart rate against my cap

My easy running has a heart rate ceiling of 131 bpm. It keeps the effort aerobic while my legs, tendons and joints catch up with the mileage. A run counts as over the cap when the average is more than 2 beats over it, or more than a fifth of the time sits above it.

91% at or under 1319% above

Under the cap. Average 126 bpm, with 9% of the run above 131.

How well the running held up

I compare how efficiently I was running early in the run (speed for each heartbeat, corrected for hills) with the late part. The drop was 5.4%. Over 5% is what I call a late fade. That band is a coaching convention, not a validated threshold.

A sustained drop began around 2h 20m into the run.

Late on, pace slowed 2.1% and heart rate rose 3.5%.

Efficiency across the run against the early baseline (100%). The dashed line is 95%.

First half against second half

Comparing pace against heart rate in the first half with the second, the gap was 7.5%. Under 5% is the convention I coach to: it suggests the aerobic side coped with the length of the run. It says nothing about why.

Mile by mile

Mile	Pace	Avg HR	Climb
1	13:08	115	46 ft
2	13:30	122	7 ft
3	13:38	125	7 ft
4	13:41	125	0 ft
5	13:36	122	0 ft
6	13:34	123	0 ft
7	13:23	124	0 ft
8	13:33	124	0 ft
9	13:49	116	13 ft
10	13:50	125	0 ft
11	13:55	129	13 ft
12	14:24	128	13 ft
13	14:23	127	7 ft
14	13:39	118	20 ft
15	13:39	138	52 ft
0.30	13:56	145	20 ft

Heart rate in orange is above the 131 bpm cap.

Figures from my watch via Intervals.icu, read by the Ultra Event Training app. Heart rate from a watch is a proxy for effort, not a lab measure. The efficiency figures compare speed per heartbeat early and late in a run with a hill correction; they show what changed, not the cause.