Running in heat and tough conditions

It can cost you a lot, and it depends on how hot it is, what you are used to, and the humidity. As a rough guide from the research, the marathon's sweet spot is cool, around 7.5 to 15 degrees WBGT, and above that you lose roughly 0.3 to 0.4% per extra degree, with real-world slowdowns running from about 5% to north of 20% in hot, humid races. Humidity bites harder than temperature on its own, and slower runners lose more because they are out there longer. The heat tool gives you the number for your conditions.

Yes. As a guide, add something like 25 to 30 seconds per km when it is genuinely hot, make that adjusted pace your target, and use effort and feel as the check on top. The heat tool gives the precise number for the day you are running.

Not really, unless you are pointing at a race you know will be hot, in which case work toward it. For most people, knowing how heat affects you and adjusting on the day is enough, and you pick up some acclimation for free just from training through a warm season.

Pull the pace back and keep yourself as cool as you can. The moves with the best evidence are pre-cooling before the start and cooling from the outside during the race: cold water on the spots that dump heat fastest, your palms, face, neck and wrists, plus something like a cooling headband. Ice on the skin and cold drinks feel great, but the performance evidence for slushies is thin, so do not lean on them.