A blog post from Brian Mac, Including a handy calorie calculator
For every physical activity, the body requires energy and the amount depends on the duration and type of activity. Energy is measured in Calories and is obtained from the body stores or the food we eat. Glycogen is the main source of fuel used by the muscles to enable you to undertake both aerobic and anaerobic exercise. If you train with low glycogen stores, you will feel constantly tired, training performance will be lower, and you will be more prone to injury and illness.