Let’s say you currently race a 22:45 5k and a 47:30k. For training you run 7k three times a week at a comfortable 4:55/km (7:55 mile pace). You know you could probably go a bit faster but you remember reading that it’s a bad idea to do all your training runs at a race pace. Are you going to be faster in your next race? Maybe, maybe not, as there are other factors such as cross-training and nutrition involved. However, by changing from steady-state runs to intervals you can train for the same amount of time (or even less) and see more progress.
What is an interval? Basically, as the image at the top of this article shows, it means you have periods of running hard and periods of recovery within the same race. There are specific types of interval runs (tempo, fartlek, progression, hills, etc.), but that’s a topic for another day. Essentially, interval is a broad term for any time in you run harder than is comfortable with a (moving) rest afterwards that brings the heart rate down to baseline.
What’s so special about intervals? Athletic improvements come about from stress on your physical system, whether it’s the breakdown of muscle fibers or high demands on your circulatory system. Stress on the system leads to all kinds of physiological adaptations that equate to better performance, whether it’s the increase in muscle fibres, increase in blood volume, improved oxygen exchange, increase in red blood cells, increased lung capacity, etc.
The key is: if you are in good shape and you don’t stress your system you will see limited improvements to fitness. However, it is also simply unsustainable to go as hard as you can for as long as you can each workout. Sure, you will stress your system, but to a degree that will require a very long recovery period (days, or weeks if you injure yourself). What an interval run solves is that with short bursts you can add the stress, but the periods of recovery prevent over-exertion.
So, from the opening example, the runner is doing 21k per week. Rather than doing 3 runs of 7k at steady pace, one 5k run with intervals, one 6k tempo run, and one 10k easy run would lead to better results from the same distance.