Also, the electric battery recharges itself while driving downhill and during braking. Therefore, if the car is used wisely, you will be able to go longer on one tank of gas. This is exactly how the body works, and this is where it so often goes wrong, resulting in overtraining, decreased fitness and health levels, fatigue, burnout, loss of motivation and, believe it or not, even weight gain!
So let’s compare our bodies to the hybrid car above. At all times, our bodies utilize energy, even while sitting in a couch. And the energy we consume is always a mixture of both fats and glycogen. We have to use that energy wisely to keep ourselves running for the long haul. The general rule is that when training or just existing at lower intensity levels, the mixture of energy used is predominantly coming from fats. While increasing cycling endurance, we all have a specific heart rate at which this mixture shifts to predominantly glycogen (carbohydrates stored in the liver and the muscles). The thing about these energy sources is how much fat and glycogen your body has in stock. This is exactly why everyone, especially fit individuals with performance goals, have a real need for endurance training.
For example, a man who is 5 ft., 8.5 in. (174 cm) tall, weighs 154 lb (70 kg) with 12% body fat, has 94,500 calories available from fat. This same man will only have about 1,800–1,900 calories available from glycogen. The 94,500 calories available from fat is pretty much a limitless pool of energy you‘d rather use before spending the more scarce 1,900 calories from glycogen. Once you have used these 1,900 calories, you will be slowed down or even stopped, just like our hybrid car that has run out of gas!
Now, you could eat while increasing cycling endurance to keep this stock of glycogen up. Unfortunately, this will not restore your glycogen completely. Your body can only take up a few hundred calories an hour before your digestive system kicks in heavily and slows you down. Training at too high a heart rate will also eventually slow you down and make you stop.
These are some of the reasons why endurance training is needed for everyone, not only for new riders or people looking to loss weight. If you wish to be a healthy person and perform well at endurance sports (especially efforts over 90 minutes), you will not perform your best without sufficient endurance training.