Endurance running uses a combination of three primary energy pathways to cope with the physiological demands of racing. These pathways are: the lactate system, the anaerobic system and the aerobic system.
An endurance runner will primarily use the aerobic system but it’s important to remember that these systems are interconnected and various distances use each system to a greater or lesser degree. None of these energy systems can operate in total isolation from the others.
A good indicator of which system you’re using is heart rate, and there are several methods to determine what your maximum is. The most commonly used crude method is 220 minus your age, though a study by Tanaka has found issues with older and female athletes, and other methods are available like heart rate monitors.
Once you have this data, you can determine what percentage of your maximum heart rate you’re using. If you’re running between 80%-105% then you’re operating in your anaerobic system; as you start to drop below 80% your efforts become more aerobic.
If you’re not a fan of using heart rate monitors or prefer to be technology free a good way to think about energy systems is that lactate efforts (on a running track) would be no more than 20-60m at maximal effort, anaerobic efforts would range between 60-600m at maximal effort and anything over that would primarily use the aerobic system.
When it comes to pacing efforts and not setting off too quickly in races, out-and-backs can be of great benefit. These efforts are done to time rather than distance. Simply find a field or loop course and run 3mins in one direction, stop where you are and recover for 2mins, then run back the opposite direction for 3mins, repeat this up to 4 times (8 reps in total). The purpose of this session is to always find yourself back exactly where you started.