The morning air has noticeably changed this week. After months of running through heavy summer humidity and high dew points, late September has finally brought that cool, crisp autumn breeze.
Running in lower temperatures immediately transforms the entire experience. Breathing feels effortless, the cardiovascular strain drops, and legs turn over with far less friction. Over the past couple of weeks, that seasonal shift showed up clearly in the data: my estimated VO2 max has been steadily climbing back up as the body no longer spends massive amounts of cardiac output simply shedding heat.
With an autumn half marathon race coming up next weekend, this was also a week for conscious restraint. I deliberately kept the weekly volume controlled at 71.1 km across 7 consecutive runs, staying sharp without accumulating deep fatigue.
Weekly Breakdown: September 14 – September 20#
Here is how the week stacked up in the log:
| Date | Distance | Pace | Avg HR | Effort & Context |
|---|---|---|---|---|
| Sep 14 (Mon) | 11.88 km | 6'05’’ / km | 139 bpm | Opening the week with a steady aerobic rhythm |
| Sep 15 (Tue) | 10.05 km | 6'09’’ / km | 136 bpm | Smooth baseline volume, relaxed breathing |
| Sep 16 (Wed) | 11.33 km | 6'02’’ / km | 139 bpm | Crisp morning breeze, effortless leg turnover |
| Sep 17 (Thu) | 5.73 km | 6'11’’ / km | 133 bpm | Mid-week recovery jog, lowest cardiovascular load |
| Sep 18 (Fri) | 10.20 km | 6'08’’ / km | 138 bpm | Smooth aerobic run, zero muscle soreness |
| Sep 19 (Sat) | 16.20 km | 5'57’’ / km | 144 bpm | Longest run of the week — dipping sub-6'00’’ with heart rate comfortably capped |
| Sep 20 (Sun) | 5.74 km | 6'06’’ / km | 136 bpm | Gentle shakeout jog to close the week fresh |
- Total Distance: 71.1 km
- Average Pace: 6'04’’ / km across the entire week
- Average Heart Rate: ~139 bpm — locked squarely inside Zone 2
The Autumn VO2 Max Rebound#
During peak summer, running pace-to-heart rate metrics can be deceptive. When ambient temperatures hover around 28°C to 30°C with high humidity, the body faces competing circulatory demands: the heart must pump oxygenated blood to working muscles while concurrently shunting large volumes of blood to the skin capillaries for evaporative sweating.
That dual demand creates noticeable cardiac drift — heart rate ticks upward 10 to 15 bpm higher for the exact same pace. Because watch algorithms estimate VO2 max by comparing running speed against heart rate response, summer runs frequently register as “declining” or stagnant fitness.
As soon as autumn morning temperatures drop below 20°C, that thermal tax disappears. The heart returns to its baseline stroke efficiency, heart rate stabilizes at identical paces, and the estimated VO2 max trend line climbs right back up. Seeing that rebound in the numbers is a reassuring reminder: the slow, sweaty summer miles were doing their job all along.
Holding Back: Pre-Race Discipline#
When the weather turns ideal and every step feels springy, the strongest urge is to push: run 22 km on the weekend, chase faster splits, or test race pace early.
With the half marathon only a week away, I chose deliberate restraint:
- A controlled long run: Instead of pushing past 20 km, Saturday’s run was capped at 16.20 km. It was the fastest run of the week at 5'57’’ / km, yet average heart rate stayed at 144 bpm (well below threshold). I finished feeling like I could easily have run another 10 kilometres.
- Two short recovery buffers: Thursday (5.73 km, 133 bpm) and Sunday (5.74 km, 136 bpm) provided restorative recovery jogs. They preserved the daily streak and neuromuscular rhythm without taxing the tendons or calves.
- Fresh legs for race week: Closing at 71.1 km leaves the body primed rather than depleted.
Next week is race week: a couple of short shakeouts, plenty of sleep, and stepping up to the starting line rested and eager to race.
Previous log: September 6 Xi’an Ancient City Wall Loop and Mount Hua — Running the wall perimeter and climbing 1,963m.