Improving as a runner involves 3 key adaptations: 1. increasing your muscle oxygen uptake and utilization, 2. enduring higher workloads for longer, and 3. exerting higher force on to the ground.
Improving muscle oxygen uptake comes from exercising at your Aerobic Threshold, known as your base-building intensity. This is where your body relies proportionally the most on Fat as a fuel source, and your muscles have as much oxygen as they need to become more efficient at consuming it. You can sustain this effort level for long periods of time at a relatively low energy cost, which gives your muscles the time they need to adapt. They get stronger and more efficient at utilizing fat as a fuel source. Improving your Aerobic Threshold is an important building block to sustaining faster paces for longer periods of time.
Many people go too fast on their easy days. Doing your base building at the Aerobic Threshold ensures you get all the benefits, while minimizing the overall stress on your body. This is key when you add workouts that target your Anaerobic Threshold - improving it translates to sustaining faster paces. Many people know this workout as a tempo run, but there is a lot of confusion as to why and how it should be done. By training at and slightly above your Anaerobic Threshold, you are strengthening your endurance muscles at the limit of their current ability. If you’re running Tempo too far below your Threshold, then you won’t see improvement. If you’re running too far above, then you won’t last long enough to realize the training effect. Having a strong aerobic base before performing these workouts is critical. You won’t fatigue as quickly, which gives your muscles valuable minutes to adapt to the stress of the higher effort workouts. Over time your muscle fibers will strengthen, and your ability to create and consume lactate as a fuel source, will make what was once hard, much easier.
The third goal is achieved through sprint workouts and/or strength training. These workouts overload your muscles for short periods of time, which helps to train your body to recruit muscle fibers that are neglected at lower intensities. As your ability to apply higher forces improves, it becomes relatively easier to maintain a given sub max speed. When you overload your muscles in these workouts, it's critical to give them adequate rest in between sets. Resting length should be at least 2 and up to 4 times longer than the active set itself.
To become a faster runner, you need to improve your muscles’ ability to transmit and sustain higher forces onto the ground. You do this by making your muscles stronger and more efficient by working at your Aerobic and Anaerobic Thresholds, doing sprints, and strength training. If you’re spending most of your training somewhere between your base-building intensity and your Anaerobic Threshold then you will likely plateau, or see very slow improvement, because those workout intensities are basically base-building workouts that are harder than necessary. By targeting your ideal effort levels, you will see much faster improvement.