The danger zone for experiencing some sort of heat related complication usually starts once the temperature rises above the low seventies with a humidity of over 50%. The risk of suffering from heat exhaustion also depends on the distance you plan to run and the shape you are in. But once the temperature starts hovering around 90 degrees or greater, all runners no matter what shape they are in or the distance they are running, must take precautions.

The best weapon to use against the heat is water. Drink plenty of water during the hours leading up to your run, but try to avoid gulping down more than one full glass within 30 minutes before going out. The heat will be tough enough to deal with , don’t make the situation worse by adding cramps to the run from drinking too much water prior to your training.

Beside down your throat, the second most important area to apply water is on the back of your neck.

The area behind your neck is the most efficient cooling connecting point to the rest of your body. A cup of water splashed behind your neck can go a long way on a hot days run.

Heat Alert! Be Careful.
Heat exhaustion is a milder form of a heat-related illness. With heat exhaustion your core temperature and heart rate increase, and you may experience exhaustion, dizziness, nausea, headache, or shortness of breadth.  The good news: by taking action now, you can prevent heat fatigue during your long runs.
Causes (compounded by hot, humid conditions)
1. Running continuously on long runs (or walk breaks too infrequent)
2. Running slightly too fast on long runs during adverse conditions

The consequences of running too fast:
You'll usually feel great during the first half of the your long ones if you're running one minute slower than you could run on that day. Unfortunately, this pace is too fast for hot and humid conditions and will dramatically increase leg fatigue and slow down your marathon performance.

Prevention:
1. Adjust for warm weather by running at least 2 minutes per mile slower than you could have run that distance that day. Account for heat, humidity, hills, and other adversities. If you're in doubt, run 3 minutes per mile slower, as I do. You'll get the same endurance from the long run running slowly, as you would when running faster. Slow long ones will dramatically reduce leg fatigue, and decrease the chance of heat fatigue.
2. Take walk breaks more often when the heat and humidity are high, and as the long runs get longer. If you're walking one minute every six minutes at first, shift to 1 in 5 when the long one reaches 18. By the time you're doing your 21 miler, drop to 1 in 4. If it is extremely hot and humid on the 26 (and longer) run (s), walk 1 minute every 3 minutes at least during the first 13 miles of the run. By combining this with a pre-dawn start, you'll minimize leg/heat fatigue.
3. Drink at least 6-8 oz of water every hour you are awake. Reductions in heat fatigue buildup are realized by running during the coolest time of the day (before sunrise), drinking cold water, and pouring it over yourself throughout the long runs. Avoid salt, alcohol, and caffeine.

How to stay Cool
1. Slow down early- take those walk breaks early and often.
2. Wear lighter garments and not cotton- several materials will wick the perspiration away from your skin: coolmax, polypro, etc.
3. Pour water over yourself - pour water on your head, or even on you light coolmax (or similar material) singlet.
4. Runners Don't wear a hat. Hats keep the heat from being released through one of the best vents you have-the top of your head! Wear sunglasses to protect your eyes from the sun, and sunblock.
5. Drink cold water- it leaves the stomach quicker and it produces a slight physiological cooling effect- and even greater psychological cooling effect.
6. Drink water the day before to be well hydrated for Saturdays training.
7. Take a dip or a shower- take a break for a dip in the pool or a cold shower on hot days!
8. Don't eat a big meal- Eating too much (especially protein and fat) will put extra stress on your system when you exercise.
9. Eat an adequate meal of carbohydrates the night before. Pasta, Potatoes, Rice etc. This will keep you fueled up for the next mornings Long run/walk. You’ll need the energy to get you through the longer distance as the weeks advance.