Carry several bottles of water, and if temperatures are quite toasty, freeze one bottle (or even two), to carry in your pants or vest! Not only will you have cold water to drink as it thaws, but the chill of the frozen bottle will help keep your body cooler! Have occasional high-carbohydrate/protein snacks. Keep a cell phone handy in case you require assistance. Always notify local police of your walk and route.
To a long-distance walker, starting out in the coolest hours of the day gives you a head start, and as the day warms, drinking plenty of non-sugary liquids- especially water, and keeping your head covered with a hat is essential. Seek shady areas, especially during the hours when the sun is highest in the sky, usually from around 11 a.m. until 2 or 3 p.m., depending on the season. Plan ahead for safety.