We previously went over some basics of anaerobic exercise, now on to the aerobic portion.
3. Glycolysis - Aerobic
The burning of glycogen using oxygen can last much longer than without...but how long? Well, it's going to be directly related to how much glycogen you have stored in your muscles. So when you workout, you need to eat carbohydrates to replenish these storages. If you don't, you will feel fatigued, as your body can't produce energy at the rate you are used to. That's why you'll see people eating all sorts of sugary stuff when doing multi-hour endurance activities. Gatorade came to signficance partly due to showing that by giving carbohydrate calories to people during exercise, the participants could perform the exercise for longer.
In Hiking Terms
A main energy system to use when hiking. When you are breathing hard, you are using this pathway. Breathing hard = using a lot of oxygen! You'll use it all the time when doing moderate + hikes. The key, of course, is to make sure you don't deplete your glycogen stores before you finish the hike (or at least the hard part). This is done partly by bringing some carb-dense food, and partly by hiking at an intensity that also uses #4 below.
Again very important. You want to be able to take in a lot of oxygen for your body to use. Although it seems that you are limited by your lungs when working and breathing hard, it almost always comes down to the fitness of your heart and muscles. Training at intensities that you can only do for less than 1 hour will train your heart to pump strongly, and train your muscles to process oxygen for glycolysis more efficiently.
So training to get stronger at hiking is based around switching between aerobic and anerobic glycolysis. Mixing up intensities that you can hold between 5 and 20 minutes will sufficiently tax the cardiovascular system. It will help push the AT up higher, so you can hike at a higher intensity for longer - as long as you have the glycogen reserves!
4. Fatty Acid Metabolism - Aerobic
Uses oxygen and fatty acids to produce ATP. Burning fat will not give you as much energy / time as you need for strenuous exercise, so your body uses it more at lower intensities. I will not get into the discussion about thinking this means you should exercise at a lighter intensity to burn more fat, but basically it's not true. You want to burn the most calories to lose weight.
In Hiking Terms
You'll be using this energy pathway during hiking, especially for the longer ones. Unlike aerobic glycolysis, you won't run out of fat to use for energy. You just can't go as fast when relying on it. That's why when you are planning to go far, you have to go slow enough to use a good amount of your energy from fat. You'll plan on the rest coming from carbohydrates, but you may have only 1000 - 1500 kcal of glycogen available, so plan to spread it out over a few hours!
While some may argue specifically about improved ability to extract ATP from fat, this is not an important system to focus on when training. Basically, it's not intense enough. It's not pushing your heart to pump oxygen, nor pushing your muscles to use it. You'll use it when doing high-intensity intervals - in the rest portion. Otherwise, don't expect to exercise at this intensity and gain significant improvements in fitness.