She's also learning what challenges she'll face during the marathon and figuring out how she'll cope with them. To prevent your body from overheating during long runs, you need to continually take in fluids and carbohydrates to replace your body's depleted resources. First and foremost, all runners need to take in plenty of fluids. Even slight dehydration can affect your performance. Remember, once you feel thirsty, you're already dehydrated.
Severe dehydration can cause dizziness, headaches, nausea and a rapid heartbeat.
The American College of Sports Medicine recommends 5 to 12 ounces every 15 minutes during physical activity. A good rule of thumb is that you should replace what you lose. To figure it out, weigh yourself before a training run, then afterward. Another way to tell that you're properly hydrated is if your urine is almost completely colorless. Water alone is fine for shorter runs, but once you've been running an hour or more, you also need carbohydrates.