Carbohydrate is a macronutrient found in plant based foods e.g. vegetables, fruits, grains and legumes. Carbohydrate is stored in the liver and muscle as glycogen for readily available energy, which is particularly handy during exercise. However, these stores may be drained by long duration exercise e.g. >90-120 minutes. It is therefore important as an endurance exerciser to build up glycogen stores before endurance training and to use strategies to boost the amount of carbohydrate available to provide energy during particularly long sessions.
Endurance exercisers are recommended to consume 5-8g of carbohydrates per Kg of body weight the day before a rigorous training session to increase glycogen stores. This would look like a 70Kg person eating 350-600g of carbohydrates. Start out by trying to have 2 cups of high fibre, nutrient dense complex carbohydrates with each meal in addition to less starchy vegetables. When possible, consume more simple, easy-to-digest carbohydrates two hours before training. The amount here will depend on your ability to digest these foods close to training.