1) It depends.
The duration and the intensity of the exercise coupled with individual energy metabolism decide how much you glycogen is going to be used. When exercising, you use all energy systems all the time, so you burn fat and use muscle and liver glycogen all the time. But the intensity and length of the exercise is important. On low intensity, like an hour or more of aerobic exercise (50-65% or so of Vo2Max) will use mostly fat stores. When the intensity rises, so does the use of muscle glycogen. It doesn't take time, it takes intensity. Your fat storages will last for hours and hours, if the intensity is low enough. When you crank up the intensity, more glycogen is getting used. Depending on how hard you are going and how well you have eaten, glycogen stores will last everywhere from half an hour to two hours or more.
Interval training will deplete your glycogen storages more than low intensity aerobic exercise. 2 hour exercise will most likely use some muscle glycogen (again, depending on the intensity). I can't tell if you are draining the storages with this information. It's more like how do you feel? There's a special feeling when your body turns to more fat-based metabolism while doing higher intensity cardio. Your performance will decrease and it will hit you mentally as well.
2) Eat carbs and protein before and after the workout. A good meal 2-3 hours before the exercise is good for pre-workout. Then if you have more depletion on glycogen storages, hit a small amount of carbs (30g is enough, but you can do more) and some protein post workout to start your recovery process, then eat a big meal an hour or so afterwards. If the exercise wasn't that intense, you can just eat a meal like 45min - 1h after the exercise. No stress. You are training only once a day, so instant recovery process isn't necessary, just eat enough carbs around your workouts and you will have enough glycogen in your storages.
When competing, if the competition lasts for more than 90 - 120min, take some fast-acting carbs slowly during the competition to keep your glycogen going.
Physical Preparedness Coach
Co-Owner of UniFit Oy.