Most people who move to Bangkok from a cooler country notice the same thing within a week: a run that felt easy at home now leaves them soaked, flushed and oddly exhausted. Nothing is wrong with your fitness. Your body is simply spending a large share of its effort on cooling instead of on the work you asked it to do.

Why it feels so hard

When the air is hot and humid, sweat does not evaporate quickly, so it cools you less. Your heart pumps more blood toward the skin to shed heat, which leaves less available for working muscles. The result is a higher heart rate at the same pace and a stronger sense of effort. Bangkok adds a twist: humidity often sits above 70 percent even in the early morning, so the "cool" hours are only relatively cool.

Give acclimatisation two weeks

The good news is that the body adapts. Over roughly 10 to 14 days of regular, moderate exposure, plasma volume rises, you start sweating earlier and more efficiently, and your resting core temperature drops slightly. You can help the process along by keeping sessions shorter and easier at first, then extending them. A useful rule for the first fortnight: cut your usual duration by a third and ignore pace entirely.

Train by effort, not by numbers

Pace and power targets set in a temperate climate will mislead you here. Use perceived effort or heart rate instead. If you could hold a conversation at home at a certain pace, aim for the conversational effort here, whatever pace that turns out to be. Many runners find they are 30 to 60 seconds per kilometre slower for the same effort during the hot season, and that is fine.

Practical timing

Before 7am and after 6:30pm are the most comfortable windows outdoors. Lumphini and Benjakitti parks get busy early for a reason. On days when the heat index is extreme, move the session indoors or swap it for strength work in an air-conditioned gym. One missed run will not matter; heat illness will.

Know the warning signs

Headache, nausea, goosebumps or chills while hot, confusion and stopping sweating are signs to stop immediately, get into shade or air conditioning, and cool down with water and fanning. Heat illness can escalate quickly, and people who are fit are not immune; they are often the ones who push through.

Small habits that help

Wear light, loose technical fabric rather than cotton. Pre-cool with a cold drink or a cold towel on the neck before you start. Weigh yourself before and after a long session once or twice to learn how much you sweat, then plan to replace most of it over the following hours. Keep a towel and a spare shirt in your bag, because the BTS will be cold.

Give it a month. Most people find that by their second hot season, training here feels almost normal.