How much should I drink on a Long Walk?

So you have found you quite like exercising and are thinking about upping things a level, but aren't sure about how much fluids you need to take with you.  It is always good to prepare and here are some guidelines to help you get it right.  It is better to be over prepared rather than under prepared as your health and well being are important.  Whatever you do, enjoy it and reap the rewards of your hard work.

Keeping enough fluids and salts in your body during a long distance walking event such as a marathon, 20-miler, or half-marathon is crucial to finishing feeling good. Getting hydration right can prevent spending time in the medical tent getting an IV or a scary trip to the emergency room. Don't end your race with a ride in the sag wagon or an ambulance—learn to drink right.

Dehydration

Dehydration is losing too much fluid from your body. You'll lose fluid due to perspiration as well as simply due to breathing.

Signs of dehydration include dry mouth, fatigue, dizziness, stomach ache, back pain, headache, irritability, and decreased urination. What urine you pass may be very dark yellow or gold.
The basic rule of thumb is to start a walk having had 16 oz. of water (a pint or a half liter), and then replenishing with a cup of water every 15 to 20 minutes. That is about a water bottle full an hour, about a half liter or a pint. End your walk with a big glass of water. While you are walking, drink whenever you feel thirsty as well. 

Hyponatremia (Loss of Sodium or Body Salt)

Sweating removes both water and salts (electrolytes) from your body as you walk. Depending on the humidity, you may not notice how much you are sweating. For walks of over an hour, replenishing your electrolytes with sports drink is also important to prevent hyponatremia. Signs of hyponatremia include nausea, headache, cramps, confusion, slurred speech, bloating and swollen hands.
It is recommended to drink one bottle of sports drink for every two bottles of water. Eating salty snacks such as pretzels during long walks is another source of salt. You might even consider using one of those little packets of salt you see at fast food restaurants—some races hand those out on the course.
On long distance courses such as the marathon or half marathon, walkers and slow runners are at risk of drinking too much plain water and flushing out their body salts, resulting in getting dangerously sick with hyponatremia. Let thirst be your guide rather than pushing too much water.
You can also be at risk after you finish your walk. Replenish fluids, but don't overdo plain water. Ensure you are balancing fluids with salty snacks or drinking electrolyte-replacement sports drinks.

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