When it is hot, your body sends blood to the skin to shed heat. That is blood that is not going to your working muscles. Your heart compensates by beating faster, which means the same easy pace now costs you more effort than it did in April. Nothing is wrong with you. You are paying a tax. The number that matters is not the temperature. It is the dew point. Dew point tells you how much moisture is in the air, and moisture is what stops your sweat from evaporating. Sweat that does not evaporate does not cool you. It just soaks your shirt. A rough guide: Below 55 — you will barely notice it 55 to 65 — you will feel it, expect to slow down 65 to 70 — hard, and it will feel harder than it should Above 70 — cut the run short or move it indoors It is entirely normal for the same effort to cost you 30 to 60 seconds a mile at a high dew point, and sometimes more. What to actually do: Run early or late. Between roughly 11am and 4pm you get peak air temperature plus direct sun, and the sun is often the bigger problem. Run by effort, not by pace. Use the talk test. If you cannot speak in short sentences, you are going too fast regardless of what the watch says. Drink before you go, not just during. You cannot catch up mid-run. Give it time. Heat adaptation takes about ten days to two weeks of consistent exposure. The first few hot runs are the worst ones. Stop immediately if you get chills, goosebumps, or stop sweating. Those are not signs that you are toughing it out. They are signs that your cooling system is failing, and that is a medical situation, not a training one. Your slower summer paces are not a step backward. Most runners find their fall races feel surprisingly easy after training all summer in the heat.