500 calories in carbs = eating 125 grams of carbs in one sitting , I don't think it's a good way to start a day, especially if they are junk carbs such as cereals and fruit juices...If you're going to eat a 500 calorie breakfast then at least split those 500 calories into protein, carbs and fats...Have some eggs, with hash browns and some wholegrain toast with butter 1 hour before your ride.