Drink 10 - 16 oz. Then drink every 20-30 minutes along your walk. You can tell if you end up dehydrated after your walk if your pulse rate remains high and your urine is dark yellow. Thirst doesn't tell the whole story, drink during your walk whether or not you feel thirsty. Avoid drinks with a high sugar concentration.
Water is great.
Sports drinks are worth a try. Make your own shade. Wear a hat with a visor or a desert-cap with flaps to shade your neck. Wear sunscreen to prevent sunburn, skin cancer and wrinkles. Wear sunglasses that filter UVA and UVB to protect your eyes.