To ensure optimal energy levels during high intensity training, aim at consuming ½ your lean body weight in carbohydrate grams for every hour prior to starting your workout. For most athletes, this equates to 45 to 75 grams of carbs per hour.
During training, aim at ½ gram of carbohydrate and up to 1/8 gram of protein (desirable when training for Half Ironman and Ironman distance races) per pound of lean body weight. Again, an hourly dose of 45 to 75 grams of carbs is generally advised.
Gels are a the most convenient way to refuel during training. Other alternatives include chews, energy bars, and carbohydrate-infused sports drinks.