Your muscles primarily use a mixture of carbohydrate and fat to fuel exercise. At lower intensity exercise fat becomes the primary fuel but as you increase exercise intensity your muscles rely more and more on glycogen. That's what understanding how much glycogen is stored and the rate that it is metabolized is important for every endurance athlete to know. When muscle glycogen stores are depleted you "bonk" or "hit the wall". Glycogen use is highly dependent on exercise intensity.
The table below show the relationship between intensity and glycogen utilization. At 100% VO2 Max glycogen stores can be depleted in as little as 8 minutes. On the other hand at an exercise intensity of 70% VO2 Max you can probably exercise for 150 minutes before depleting your glycogen stores. To place this in perspective, increasing exercise intensity from 70 to 80% VO2 Max increases glycogen metabolism four-fold.
There are a number of strategies you can implement to extend your exercise time of endurance. Training makes your muscle metabolically more efficient. However the simplest strategy is to make sure you consume the right combination of fluid and nutrition during your workout which can significantly increase both the quality and duration of your workout.
The bottom line - there is no simple answer to how far you can go on a tank of muscle fuel.