So, here’s what I found about current recommendations on hydration for a marathon. I thought you might like it since it is very current.
http://walking.about.com/od/fluids/a/drinking2006.htm
The International Marathon Medical Director's Association revised guidelines for drinking and fluid intake for walkers and runners at endurance events in May 2006.
What Should You Drink?
For a workout of 30 minutes or more, they recommend drinking sports drink, and not diluting it or alternating sports drink with water. The carbohydrate and electrolytes in the sports drink helps the body absorb water faster, and provides energy for the body. If you dilute the sports drink, you decrease the benefits.
In my experience, many walkers are likely to try to ignore this advice in order to take in fewer calories. During a marathon or race, they should drink carbohydrate-containing sports drink for performance. For walking workouts, they could use a low-calorie sports drink to replace salt without adding calories.
However, the IMMDA also recommends that during a marathon, participants drink whichever beverage most appeals to them, relying on their body to know whether they need more sodium or more water.
Event directors need to have both water and sports drink available at water stops. I think this is good advice for walkers on long walks as well - have both available to you and drink whichever appeals to you at the moment.
How Much Should You Drink?
There are dangers in drinking either too much or too little. Drink too much and you risk hyponatremia - low blood salt level and fluid overload. Drink too little and become dehydrated. The needs will vary with many factors: the weather, your body's reaction to the exercise demands, sweat rate, etc.
Weigh Before and After: Weighing yourself before and after exercise can often help you know whether you are drinking too much or too little. The guidelines say: a weight loss of more than 2% or any weight gain are warning signs that justify immediate medical consultation and indicate that you are drinking too much or too little.
Drink to Thirst
Erase the old advice that you can't rely on thirst. New evidence says that thirst is the best protection for athletes when it comes to drinking the correct amount.
Drink when you are thirsty.
Don't drink if you aren't thirsty.
Don't drink at every water stop at an event just because it is there or your companions are drinking.
Rely on your thirst unless you discover it is leading you wrong, from weighing yourself before and after a workout.
Drinking Guidelines For Walkers and Slower Runners
No more than 1 cup of water per mile is a good rule-of-thumb for walkers and slower runners - anyone who takes more than 4 hours to complete a 26.2 mile marathon, or a pace of greater than 10 minutes per mile.
Your weight determines the range - a half-cup if you weigh 100 pounds and a full cup if you weigh 200 pounds.
The slower you are, the less you should drink. While a fast runner may need 4 liters of fluid for a marathon, a walker or slow runner needs only 2.5 to 3 liters for the entire event.
Thirst may not kick in as fast if you are in extreme heat and not yet acclimated to it, or in cold weather, or if you are over 65.
Calculating Your Fluid Needs
Your needs may change based on the weather, your conditioning, and other factors. IMMDA provides this method of determining your fluid needs:
One Hour Test
Weigh yourself nude before the walk or run.
One Hour Test: Walk or run or alternate walking/running at race pace for one hour, just as you will do during the race. IMMDA recommends one hour to get the sweat rate you will have during the endurance event.
Write down how much you are drinking, in ounces, during the 1 hour walk or run.
Weigh yourself nude after you finish the 1 hour walk/run. Subtract from starting weight. Convert the difference in body weight to ounces (multiply pounds by 16).
To determine hourly sweat rate, add to this value the volume of fluid consumed (from Step 3).
To determine how much to drink every 15 minutes, divide the hourly sweat rate by 4. This becomes the guideline for fluid intake every 15 min of a walk/run.
Record the weather and conditions on your test day. Do the test again on a day with different weather and conditions, so you can see how your sweat rate reacts to different conditions.
Source: Lewis G. Maharam, MD.FACSM (chair),Tamara Hew DPM, Arthur Siegel MD, Marv Adner, MD, Bruce Adams, MD and Pedro Pujol, MD, FACSM. "IMMDA’s Revised Fluid Recommendations for Runners and Walkers." IMMDA. 6 May 2006.
Tamara Hew-Butler, DPM, Joseph G. Verbalis, MD,w and Timothy D. Noakes, MBChB, MD, DSc, "Updated Fluid Recommendation: Position Statement From the International Marathon Medical Directors Association (IMMDA)," Clinical Journal of Sports Medicine, 2006;16:283–292)
