Hydration should be your #1 nutrition priority as you train.

Among other things, water is the main component of your muscles and blood; protects your joints, brain, and spinal cord; and removes waste products from your system. It is also your body's coolant, keeping you from overheating. Don't rely on thirst to tell you when to drink. Thirsty or not, drink 8-10 cups of water and non-caffeinated beverages each day and an additional 4-8 ounces every 15-20 minutes while training. Be aware that might need to carry fluids with you during your training runs or walks. Your urine should be pale or clear in color and you should urinate at least four full bladders a day.