The general advice given to anyone exercising in hot weather is to keep drinking the water often provided at water stations in an event near you.
However drinking water does not necessarily prevent you from dehydration in fact it will also upset your natural electrolytes balance and increase dehydration. You will be familiar with the taste of sweat it’s salty, if you sweat a lot you will lose lots of natural salts. These salts are regulating your natural fluid balance, in this case the longer you exercise the more salts you lose, you may begin to feel dizzy, thirsty and even delirious, if this happens you could collapse with dehydration.
What you should do is plan ahead to avoid this happening.
Heading out on a multi hour training session in hot weather plan to drink at least 500ml isotonic drink every hour, you can make this easily using around 20% fruit juice topping the bottle up with an electrolyte tablet available at most sports stores. This will top up your lost salty sweat with fresh supplies of essential electrolyte minerals.
You will notice a considerable improvement in your performance once your body is properly balanced. You could also add in fluid based carbohydrates to assist the maintenance of your glucose energy supply which may also keep you going during your event or training.