My long runs in Sydney sat around 135 bpm at 5:30 min/km. Comfortable, conversational, well within what I’d call easy. Then I moved to Singapore.
Same effort. Shorter run. Slower pace — 6:15 to 6:40 min/km. Heart rate: 155 bpm and climbing.
Nothing had changed about my fitness. Everything had changed about the conditions. And that gap between what I expected and what my body was doing forced me to rethink something I thought I already understood: what “easy” actually means.
The short answer is that I had been running Zone 3 without knowing it. Not because I was being reckless, but because the environment shifted my zones without warning and I kept chasing the same pace numbers I’d always trusted.
Before going further, it helps to define the zones briefly. Zone 2 is the aerobic effort level where you can hold a full conversation in complete sentences without pausing for breath — typically around 70–75% of your maximum heart rate. Zone 3 is the grey zone just above it: harder than recovery, easier than threshold, and deceptively productive-feeling. It’s not comfortable enough to build aerobic base efficiently, and it’s not hard enough to generate the speed adaptation of genuine quality work. It accumulates fatigue at high cost and returns less than either end of the spectrum.
Most self-coached marathon runners live here without realising it.
How the Environment Can Force You Into Zone 3 #
The common framing of Zone 3 drift is that runners are just running too fast on easy days. That’s true. But there’s a version of this problem that’s more insidious: the environment can push you there even when your effort and pace feel completely normal.
Moving from a temperate climate to the tropics is an extreme version of what happens to any runner when the seasons turn — except here, the season doesn’t turn back. When temperature and humidity climb, your cardiovascular system works harder to cool your body — diverting blood flow to the skin for heat dissipation, leaving less available for the muscles generating pace. Heart rate rises. Perceived effort can lag behind the actual cardiac load by several minutes, which means you feel fine right up until you don’t.
In Singapore’s heat, at dew points that rarely drop below 24°C, the adjustment is permanent and significant. A run that would have been Zone 2 in Sydney sits firmly in Zone 3 here at the same pace — sometimes by 15–20 bpm. If you keep chasing your old pace targets, you’re not running easy. You’re grinding.
The Zone 2 Self-Audit: What Your HR Data Is Telling You #
The self-audit here isn’t complicated, but it does require honesty.
Step 1: Find your Zone 2 ceiling. A common method: 180 minus your age (the Maffetone formula), adjusted slightly based on training history. For most recreational marathon runners in their 30s and 40s, this lands somewhere between 135 and 148 bpm. Another reference point: 75% of your maximum HR. Neither is perfectly precise — the point is to establish a ceiling that’s probably lower than what you’ve been running at.
Step 2: Pull your last ten easy runs. Check the average HR. If you’re consistently sitting at 150–165 bpm with a max HR in the 175–185 range, you’re in Zone 3. If that number surprises you, it’s because Zone 3 doesn’t feel like a mistake while you’re in it.
Step 3: The talk test. Zone 2 should allow full sentences without pausing. Not short answers — full sentences, mid-run, without feeling the need to breathe mid-word. If you’re slightly breathless, you’ve drifted above.
Step 4: Account for conditions. In Singapore’s heat, add 30–45 seconds per kilometre to any pace target you trusted in cooler weather. My Sydney 5:30/km Zone 2 pace became 6:15–6:40/km here — and even then, the HR ceiling requires discipline. On a humid morning above 30°C, the ceiling drops further.
Why Heat Fitness Is Harder to Keep Than It Is to Build #
Here’s the part that took me the longest to accept: heat adaptation is deeply asymmetric. You earn it slowly — the physiological changes (increased plasma volume, earlier onset of sweating, lower salt concentration in sweat) develop over ten to fourteen days of consistent heat exposure, and full adaptation takes months. But you lose it fast. A few weeks back in a cooler climate, or a stretch of treadmill running in air conditioning, and a meaningful portion of what you built is gone.
This isn’t a reason to despair. It is a reason to stop comparing your current HR data to what you were capable of six months ago in different conditions. The fitness is largely still there — the aerobic base you built doesn’t evaporate. What changes is the thermal load your cardiovascular system has to manage on top of the work of running. Once you accept that your Zone 2 pace in Singapore will always be slower than it was in Sydney, the data stops feeling like a failure and starts making sense.
What Cardiac Drift Looks Like in Practice #
On long runs, there is an additional factor worth understanding: cardiac drift, the gradual rise in heart rate at a constant pace as a run extends beyond 60–75 minutes, caused by increasing core temperature and mild dehydration. In cool conditions, drift might add 5–8 bpm over a two-hour run. In Singapore’s humidity, that number can be considerably higher.
This means a long run that starts at 138 bpm — comfortably Zone 2 — can drift into 155–160 bpm by the final 30 minutes, even without increasing pace. I’ve watched it happen on East Coast Park: a controlled first 10km, then a creeping HR that hits 158 before the turnaround, despite the pace barely changing. If you’re not accounting for this, your easy long runs are finishing in Zone 3 by default, regardless of how disciplined your opening kilometres were.
The practical response: target the lower end of your Zone 2 range at the start of long runs, not the ceiling. Give yourself room to drift upward without crossing into grey-zone territory.
The 4-Week Zone 2 Recalibration #
Slowing down is the easy part mechanically. The harder part is doing it while your watch shows a pace you haven’t run since your first year of running, on a route that used to take 55 minutes and now takes 70. The legs are fine. The ego is not.
After the Kyoto Marathon earlier this year, I took almost four weeks off — on and off, some easy movement but nothing structured. Coming back in Singapore’s heat, with the hills around my neighbourhood adding a second layer of cardiac load on top of the humidity, the recalibration wasn’t a choice so much as a reality. Short runs. 6:30 to 7:00 min/km on flat ground, slower on anything that climbed. Heart rate still brushing the ceiling despite the pace. There’s a specific kind of frustration in running slower than you ever have and still needing to walk a hill to stay in zone.
The reset looks something like this: for the first two weeks, set a hard ceiling at your Zone 2 maximum and enforce it — watch alert on, slow to a walk whenever you cross it, no exceptions. A route that normally takes 55 minutes will take 70. That’s the point.
By week three, start paying attention to whether your pace at the same HR ceiling is improving at all. Even five seconds per kilometre is the aerobic system responding. Week four, add one threshold session back and notice whether it feels different — cleaner, more precise — when the easy days have actually been easy.
Full aerobic adaptation takes 8–12 weeks. One month won’t complete it, but it breaks the pattern and generates early evidence that slower is working.
The Run You Feel Most Guilty About #
There is a version of this that takes time to believe: the run where you feel like you’re barely exercising — 6:45/km, walking the inclines, HR pinned at 138 — is often the highest-value run of the week. It’s building the aerobic base that pace work depends on. It’s the work that doesn’t show up on Strava as impressive but shows up in the second half of the race, when the runners who skipped it start to fade.
Zone 2 will feel embarrassingly slow for weeks before it starts to feel productive. The runners who stick with it long enough to see the pace improve at the same cardiac cost are the ones who tend to run their best marathons in the second half of the race, not the first.
Earning heat fitness is slow. Losing it is fast. The only answer is consistency at the right intensity — which means accepting, first, that the right intensity is probably slower than you’ve been running.
If you’re training in Singapore or heading into a hot summer somewhere — I’d be curious what your HR data looks like when you actually audit it. What gave it away first: the numbers, the fatigue going into hard sessions, or something else?
- Photo by Luke Chesser