For runners training to run 26.2 miles in one of the 90-plus marathons in the United States this fall, the brutally hot and humid weather of last Sunday’s Chicago Marathon was a reminder of the most vicious opponent in the sport: Mother Nature.

And while it is unlikely that the New York City Marathon on Nov. 4, or other late-autumn efforts like Philadelphia’s on Nov. 18 and Atlanta’s on Nov. 22, will suffer the same extreme conditions as Chicago’s, longer summers and warming weather trends mean that athletes should start planning to beat the heat, not just the clock.

“The best thing that people can do is to be ready to adjust to the heat, just in case,” said Mary Wittenberg, the chief executive of the New York Road Runners and the race director of the New York City Marathon.

So what can you do to minimize the chances of hurting yourself if, come race day, the weather is hot but you’re still hoping to cross the finish line?

Most experts say the important things are consuming the right amount of fluids, wearing lightweight clothing and slowing your pace, measures that can easily be forgotten in the excitement before a race.

“In a marathon, the problems of heat are multifaceted,” said David E. Martin, a heat expert and emeritus professor of physiology at Georgia State University who counsels Olympic athletes on handling extreme weather conditions. “It’s not just the temperature outside — the asphalt warms up and becomes like a Crock-Pot. Then you’ve got thousands of hot bodies sweating around you. And you’re running out of fluids, which can be awkward to drink during a race. They all compound negatively to affect performance.”

Those negative effects can range from sluggish times to cramps, fainting and exhaustion, which felled hundreds of runners in Chicago. Then there are potentially fatal effects like hyponatremia, a disruption in sodium balance often caused by excessive water intake, and heat stroke.

“Your enzymes work best at about 99 or 100 degrees, which is why you usually warm up before you exercise,” Professor Martin said. “But they shut down when you get to about 105 degrees, so you have a very narrow margin between acceptability in terms of activity and a danger point.”

Experts suggested a variety of ways to prepare for unexpected heat on marathon day.

CHECK THE WEATHER. Matthew Moran, an exercise scientist and collegiate running coach at the State University of New York at Cortland, urges his runners to research the record high and low temperatures for marathon day. The record high for Nov. 4, the date of the New York City Marathon, is 78 degrees and the record low is 25 degrees, according to Chris Cimino, a meteorologist for WNBC-TV. “The odds of it getting into the 80s are slim to none,” Mr. Cimino said. “But stranger things have happened.”

Remember that many temperature broadcasts are for the shade, Professor Martin said. “There’s a difference between the shade and the sun by about 10 to 15 degrees.”

MONITOR YOUR FLUIDS. In the days before the race, ensure you’re well hydrated by checking the color of your urine. “Straw-colored is ideal,” said Greg McMillan, an Arizona exercise physiologist and running coach. “If it’s darker, you’re not getting enough fluids, and if it’s clear, you’re probably drinking too much water.”

On marathon morning, skip the urge to wake early and guzzle water. Professor Martin said, “If you drink a ton of water two hours before you do something, your body will sense an increase in blood pressure and pee all the water away, so you’ve wasted getting up at 4 a.m.” Instead, consume energy drinks right before and during the race.

To avoid hyponatremia, drink roughly a cup of energy drink per aid station, Mr. McMillan suggested. “If you feel sloshing in your stomach, you probably have enough fluids,” he said.

DRESS PROPERLY. Light-colored, loose clothing made from technical fabrics is best, said Mr. McMillan. Pack three outfits: one for unusual cold, one for ideal weather and one for unusual heat.

Since the seams and cinching of new clothes can chafe over 26.2 miles, last-minute clothing purchases are a typical no-no for marathoners. But it is better to buy a white CoolMax singlet the day before a race than run in a black cotton long-sleeve.

Mesh caps, a favorite of the ultrarunner Dean Karnazes of California, release heat while shielding you from sun. “A hat and sunglasses can make you feel 10 degrees cooler,” said Ms. Wittenberg.

SLOW DOWN. Mr. Moran said: “In an incredible heat environment, blood is pushed away from the muscles as a cooling method. It’s harder for your muscles to get oxygen, so you have to adjust your pace. That’s the hardest thing — you have a goal in mind and you’re making a conscious decision to run slower.”

Once the temperatures start climbing above 70 degrees with humidity, most runners should slow their pace by 30 to 90 seconds per mile, Mr. McMillan suggested.

Mike Smith, a 2:19 marathoner from Flagstaff, Ariz., who plans to run the Olympic trials in Manhattan on Nov. 3, said that if he hears a forecast for the 70s, he’ll discard any minutes-per-mile or final goal and focus on simply finishing the race.

STAY COOL. “The day before a race, you want to stay inside, stay in the shade, stay in the air-conditioning,” Mr. McMillan said.

PACK ICE. The next best thing is a bag of ice. Pack some under your cap and in your shorts, suggests Mr. Karnazes, who has battled 120-degree temperatures at the Badwater Ultramarathon, which starts in Death Valley.

SEEK WIND AND WATER. “Normally you hate wind, but on a hot day it helps with the cooling,” said Mr. McMillan. So don’t run behind, or draft, another racer; run into the wind. Look for misting stations, soaking sponges and cups of water.

KNOW WHEN TO SAY WHEN. You will probably feel pain and fatigue; this is a marathon. But if you start experiencing dizziness, headaches, clammy skin, chills or goose bumps, stop at the nearest medical station. “You’ve trained, you’ve paid that huge hotel and air bill, you’ve got your friends back home you want to talk to and say ‘I ran the race,’” Professor Martin said. “But your training has never gone to waste, it’s maintaining your fitness for the next banana.”