Good Advice

Here is a sample of a article I read on the Seattle Marathon website. It is worth reading.

Race Day Hydration: How Much is Enough?
CALCULATE YOUR INDIVIDUAL NEEDS
Sweat Rate:

Sweat rates are not uniform. They vary according to genetics, conditioning (what shape you are in), and the ambient environment (how hot or cold it is). They can range from very slow rates of sweat loss to losses up to 3 liters/hour when exercising in very hot conditions; thus, you need to calculate your own sweat rate in conditions that match the day of the race. To do this, just before a one-hour training run (preferably at marathon pace), weigh yourself unclothed. When you return from your run, disrobe, dry yourself off, and weigh yourself again. The weight difference is the amount of fluid lost. (Rapid weight loss is an indicator of fluid loss, not fat loss). If you lost one pound (16 ounces) during that one hour run, you thus need to drink 4 ounces every 15 minutes to replace your losses.

SUMMARY
Dehydration has well known effects on performance and health (slower speeds and increased risk of heat illness to name a few). Overhydration can have similar detrimental effects; thus, creating a race day hydration strategy is important. Fluid replacement schedules need to be individualized as global recommendations for fluid replacement may not be optimal for individual runners of differing body types and with varying degrees of training and heat acclimatization. Remember to create a race day hydration strategy and stick to it.