Today in St. Louis the temperature got up to 38°C (100°F). It doesn't happen often here—thankfully—so I waited until the temperature got up near the peak and went for a run.
If it doesn't happen often, you've got to take advantage of it while it's there.
It's not about suffering—it's more like learning to deal with it. One day, in a race, that experience of going out in the heat and sun and humidity will still be in there, somewhere, exerting itself. It's not explicit, like thinking to yourself "remember that one time..." It's just part of the foundation that holds things up, and the foundation is a little tougher and you know you can rely on it to hold firm when you need it.
When I was training for the Western States Endurance Run in 2012, I took a weekend trip to Death Valley, drove a few miles down West Side Road where I wouldn't be bothered, and ran a few miles down there in the gravel. Not too far—about 4 miles (6 km)—just enough to get the feel of the thing. 47°C/117°F. The strange thing is not that you feel it in your head or your lungs, it's that you also feel it under your arms and on your upper legs, places that are probably also hot on a normal hot run but quietly hot, not registering any sort of sensor warnings in the brain. Heat on your upper thigh is a Something Is Not Right trigger for your brain, trust me.
What was even more difficult than running in the heat was traveling in the heat. I don't know if it's superstition or not, but my know-nothing opinion is that training consistently in the heat causes some physiological changes in order to cope with it. I want to believe that. Say, maybe the body gets more efficient at transferring muscle heat to the surface and dissipating it via sweat. (The brain obviously doesn't get smarter because it allows the training to continue.) So, on that trip to Death Valley, I also drove with the windows down and just soaked in 110°F+ for six or seven straight hours. Take advantage of the exceptional moments.
Talking about extremes is bro-adjacent, if not outright-bro. The idea is to get a feel for the edges so that you know what they're like when you get up to one in an uncontrolled contest. Some people will approach what they think are the edges with metrics and measurements, but you can't really know like that. Numbers can help you rationalize what you want to believe if you're not careful. They have their place. But they exist in a larger context when you're training or racing—how you feel that day, how the competition is moving, how the terrain is, how the weather is, how well you're processing water and food. I never train with a watch unless I'm on a new trail in the woods and I want to make sure I get out before sundown (and I don't do that anymore, really). If you run without metrics, you learn how to get a feel for you and the environment you're in. So few people seem to get that feel. Without it, you don't know what capabilities you really have, only the ones you can measure. (I'm probably rationalizing myself.)
And the idea is to do the kind of training that your competitors won't, and then being able to use that like a lever when you need to. I'm not training for any races, just keeping the machine tuned in case I want to.
Run in the rain. Run in the snow. Run in the heat. Run in the cold. Own the envelope.
Written on my athletic tape wrist band to remind myself during Western States (along with weekly training mileage in the preceding months):