Long run · Sunday, 4 October 2026 · Week 6, Durability Base
A steady long run undone by a stomach complaint
25 weeks to MDS
Twelve miles done as planned for week six, humid conditions, 774 feet of climbing. Pace barely moved across the run, 12:43 for the first mile and 12:34 for the last, with heart rate sitting at 124 then 123. The average came in five under the cap, and the fade between early and late running was under one per cent, the kind of number I call held.
The effort itself felt easy, good rhythm throughout. But I had loose stools near the end and needed the toilet as soon as I stopped. I'd taken roughly 40g of carb an hour and 1.5 litres of water, nothing unusual for a run this length. Legs were a little heavy by the finish too. I'll look at how the fuelling and fluids sat over the next few long runs before I change anything.
From my training journal
“Felt really good. Legs a little heavy at the end and GI issue at the end. Needed to go to toilet at the end. Loose stools 40g carb per hour roughly . 1.5 litre of water”
Easy effort · Good rhythm
- Distance
- 12 mi
- 19.3 km
- Moving time
- 2h 38m
- started 08:24
- Average pace
- 13:12
- per mile
- Heart rate
- 126
- average · max 148
- Climb
- 774 ft
- 236 m
- Conditions
- 10°C
- 77% 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.
Under the cap. Average 126 bpm, with 8% 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 0.9%. Under 5% is what I call held. That band is a coaching convention, not a validated threshold.
Late on, pace slowed 4.4% and heart rate fell 3.5%.
Confidence: medium (the hill correction changed this reading by about 9 points, and it is an estimate).
First half against second half
Comparing pace against heart rate in the first half with the second, the gap was 2.4%. 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 | 12:43 | 124 | 108 ft |
| 2 | 12:56 | 127 | 20 ft |
| 3 | 14:11 | 130 | 128 ft |
| 4 | 13:31 | 130 | 95 ft |
| 5 | 12:55 | 124 | 46 ft |
| 6 | 13:00 | 120 | 13 ft |
| 7 | 13:17 | 127 | 79 ft |
| 8 | 13:33 | 123 | 95 ft |
| 9 | 12:45 | 122 | 33 ft |
| 10 | 14:01 | 126 | 128 ft |
| 11 | 12:34 | 123 | 0 ft |
| 0.99 | 12:48 | 127 | 39 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.