My summer running pace got slower — my Garmin data showed I wasn't actually losing progress
By August, my pace was 27 seconds per mile slower than it had been in spring. If pace had been the only number I paid attention to, I would’ve assumed my fitness was slipping.
A closer look at Garmin Connect told a different story. My resting heart rate stayed steady, my training stayed on track, my VO2 max barely budged, and my predicted race times actually got faster once the weather cooled down.
After tracking my summer runs with a Garmin Forerunner 965, I compared them to my spring baseline. The data didn't prove the weather caused every slow mile, but it kept me from panicking that I was losing fitness just because the heat slowed me down.
Pace was the loudest, but least informative number
My spring runs averaged 8:48 per mile at an average heart rate of 143bpm. During the hottest four-week stretch, pace fell to 9:15 while average heart rate stayed at 144bpm.
Seeing this difference, instead of asking, “Why can't I hold my old pace?”, I asked, “Why does the same effort produce a slower pace in these conditions?”
The answer—the conditions were meaningfully different. My spring baseline runs began at an average of 13°C. My hottest summer runs averaged 26°C at the start, with higher humidity and more direct sun. I also carried water on long runs and stopped more often at crossings.
Garmin notes that its watches actually adjust your VO2 max and training status to account for the heat when you run in temperatures above 22°C (72°F). While it’s not completely accurate, it shows that training trends matter more than a single slow pace.
I compared effort, load, and fitness across 12 weeks
I used the same Forerunner 965, wrist placement, and heart-rate zones throughout 12 weeks. For tempo sessions and long runs, I paired a Garmin chest strap to reduce wrist-sensor lag. I excluded races, treadmill sessions, and two runs with obvious GPS errors from my analysis.
The analysis covered:
- Four cooler spring weeks;
- Four early-summer weeks;
- Four hot late-summer weeks.
Weekly runs stayed between 31 and 36 miles, with one threshold session and one long run each week. I compared pace, average heart rate, cardiac drift on a repeated 60-minute route, Garmin VO2 max, acute load, training status, heat acclimation, as well as race predictor.
I also logged sleep and how hard each run felt on a 1–10 scale. The watch could show associations; it could not determine causation or tell whether illness, dehydration, or stress was responsible for a poor day.
Five Garmin numbers made the slower pace less worrying
| Measure | Spring baseline | Early summer | Hottest four weeks |
| Average easy pace | 8:48/mi | 9:02/mi | 9:15/mi |
| Average easy-run HR | 143 bpm | 144 bpm | 144 bpm |
| Final-20-min HR drift on repeat route | +4.1% | +6.8% | +8.6% |
| Garmin VO2 max estimate | 54 | 54 | 53 |
| Average seven-day acute load | 612 | 635 | 621 |
| 10K race prediction | 43:18 | 43:11 | 43:27 |
Pace clearly slowed down, but other metrics didn’t match the decrease.
The VO2 max change was not something I treated as a real loss. Wearable VO2 max is an estimate derived from pace and heart rate, and even Garmin's heat adjustment can’t account for every variable.
Acute load remained in a similar range, and Garmin continued to label most weeks productive or maintaining. My race prediction shifted by seconds, not minutes. Together, those were better reasons to stay patient than one slower Tuesday.
Stable heart rate showed that I had adjusted the pace appropriately
The biggest reassurance was an average heart rate. I was running slower because holding spring pace would have turned easy days into moderate ones.
On one 27°C run, I forced the first two miles at 8:48 pace. Heart rate reached 153bpm before the halfway mark, breathing felt too heavy, and the session's training effect became beyond what I wanted. When I let pace settle near 9:20, heart rate returned to the mid-140s.
That run changed my thinking. Easy pace is an outcome of effort and conditions, not a target that must remain fixed all year.
Heart rate is imperfect too. Optical sensors can misread cadence, zones may be set incorrectly, and dehydration, caffeine, sleep, or illness can increase the number. That is why I looked for patterns and used a chest strap for the sessions where precision mattered most.
Cardiac drift showed the heat was affecting me
My repeated 60-minute route made the heat effect clearer. I ran it at a controlled heart rate, carried the same bottle, and avoided hard training the day before.
In spring, heart rate rose about 4% between the middle and final 20-minute segments while pace stayed fairly stable. During the hottest block, drift averaged more than 8%, and pace decreased despite similar effort.
That did not mean I was unfit. It meant the same run created more strain. I shortened two sessions, moved one long run earlier, and stopped using pace as the goal on humid days.
The value of Garmin was not telling me to push through. It was showing that the effort of a familiar route had increased.
Heat acclimation was useful context
The Forerunner's heat-acclimation percentage rose during the first warmer weather and fell when I spent several cooler days indoors. I appreciated that the watch recognized this.
That said, acclimation does not make heat harmless. I still watched weather guidance, carried fluids, chose shaded routes, and changed sessions when I felt unusually weaker.
I also avoided comparing my score with anyone else's. Garmin's interpretation depends on recorded outdoor activities and device-supported inputs. It is a personal feature, not a ranking of who handles heat best.
Training status kept one slow week from becoming a crisis
Garmin's training status combines longer-term changes in VO2 max, acute load and HRV status on compatible devices. Training readiness draws on factors including sleep, recovery time, HRV, acute load, and recent stress.
During my slowest week, pace was down another 12 seconds per mile. Training status still said maintaining, acute load was balanced, and HRV remained inside my normal range. Sleep was shorter, and readiness scores were lower after two late nights.
The sensible interpretation was not “fitness lost.” It was “training is broadly stable, but recovery is temporarily worse.” I replaced a hard Friday session with an easy run and resumed the workout two days later.
Garmin didn’t make that decision—weaker recovery signals did.
The cool-weather test was the most convincing
The most enlightening run came in early September, when the temperature dropped to 14°C. I repeated the same 60-minute route at the same heart-rate ceiling.
Average pace returned to 8:46 per mile. Heart rate drift fell to 4.5%, and the session felt stable. Garmin's VO2 max estimate returned to 54 after two more cooler runs, while the 10K prediction improved to 42:59.
One run cannot prove your fitness levels have dropped. This is why I would never use a hot training pace to change a race goal without checking effort, load, recovery, and a cooler comparison first.
Garmin can help, but it can create false certainty too
The Forerunner 965 made the analysis convenient because it kept pace, heart rate, load, recovery, and environmental context in one place. That cohesion is a real advantage of Garmin's premium watches.
The risk, though, is believing every score. VO2 max is not an exact measurement. Race predictor is not a promise. Training readiness is not a medical clearance. Heat acclimation does not quantify personal safety.
I also had to filter the data. Auto-pause distorted two city runs, one loose chest strap produced an unreadable heart rate segment, and route changes made three sessions poor comparisons. A graph is only as useful as the activities behind it.
If pace loss is sudden, persistent in cool conditions, or accompanied by chest pain, fainting, unusual breathlessness, palpitations, or marked fatigue, an explanation from a wearable is not enough. Stop and seek appropriate medical advice.
My verdict: slower summer miles did not equal lost fitness
My average pace slowed substantially through the hottest weeks. That part was real.
But steady heart-rate effort, stable training load, a nearly flat VO2 max estimate, little change in race prediction, and rebound in cooler weather all argued against fitness loss.
The Garmin did not rescue my ego by portraying a better result. It gave me enough context to train at the right effort, respect the extra strain, and wait for a fairer test.
That is the lesson I would carry into every summer: pace is just one number. Before deciding you are getting less fit, check what your heart, training load, recovery trend, and the weather say about the work it took to achieve it.
