Practice taking in small amounts of water at a time, and getting salt in fluids, and notice how your body reacts. Train in a hot environment if you can, to get a better sense of how you react to heat over time. As a general rule, try to take in one gram of sodium per hour during a long event, and build up your sodium reserves by taking 10 to 25 grams a day of salt pre-race.
Training to avoid hyponatremia:
Drink small amounts of fluids to stay hydrated, but don't overdose on water.
Take in electrolytes through sports drinks which have sodium and potassium.
Ask the race director what fluids will be given on race day.
Try to eat snack foods with salt, like pretzels or chips.
Alternate water and hot clear chicken broth to get both water and salt.
Tomato-based drinks include high sodium content.
Salt tablets should not be used unless you have used them before, know a safe dosage and know how you tolerate them.
Avoid taking aspirin, acetaminophen or ibuprofen during the race, as it may affect kidney function.