Running and Training Different Energy Systems
When programming, it's smart to include a variety of energy systems in your training.
Anaerobic exercise uses glucose instead of oxygen. Think of this as your interval training and strength training sessions. Shorter and sharper bursts of energy (under 2mins)
Training at higher intensities will improve your ability to convert available 'sugars' into energy and can delay the production of lactic acid.
So if you become better at using this energy system, you'll save some oxygen too. The handover (or tipping point) from oxygen to glucose is called your Anaerobic Threshold. Increase this threshold by training both systems. The higher your anaerobic threshold, the longer you'll be able to run using your cardiovascular system without stealing little bits from your Anaerobic stores, which makes way for better performance.
You'll be conserving glucose and saving it for a sprint finish, and the less likely you are to feel heavy in the final few kilometres of the run.
Keep in mind that the body uses different forms of energy at all times to get through a run. You're never exclusively using one or the other. It's more like a seesaw balancing act.
In summary, remember to mix up your training to:
1) Keep your training interesting
2) Develop more than just the aerobic energy system
3) Develop multiple components of fitness