Most runners train for a race with a goal pace in mind. Many will train for that goal pace by running it in varying distances and durations during their training.
Various authors, most recently and notably Matt Fitzgerald in 80/20 Running, advocate building a solid base of mostly easy running from which you can do a bit of tempo-specific running each week. This makes sense since your ability to run fast won’t matter much without the aerobic development to sustain a desired pace over your desired race distance.
Jeff Gaudette of Runners Connect takes this a step farther. He actually posits that most runners already have the desired speed to run a goal pace, that what they lack and need to develop is the aerobic and neuromusucular fitness to sustain that pace for their desired race distance.
Gaudette has a good point. Whenever you are able, go outside and run as fast as you reasonably can (i.e. don’t hurt yourself). I imagine if your pace was measured you’d easily exceed your desired goal pace.
I also imagine you won’t be able to hold that fast-as-you-can pace for very long. Running at max speed, you’ll be winded and your muscles will be neurally screaming in seconds. I’ve done max speed reps for giggles a few times, and I find the longest I can reasonably go at that intensity is about 30-45 seconds.
When we do speedwork, we’re not really training ourselves to run faster. Most of us already can run pretty fast. What we’re training is the ability to hold a given speed over a desired distance, whether that distance is 400 meters, 5K, or a marathon. (Ultra distance runners by and large have other aerobic and endurance concerns during training aside from speed)
This is why many coaches say the goal of speedwork should be economy, i.e. refining your form and taking every step as efficiently as possible, so that when you run your races you’ve honed and improved the efficiency of every step.
I realize I’m digressing a bit. I mentioned easy pace for a reason. We focus a lot on speedwork, on our goal pace, while forgetting that every goal pace has a corresponding pace at other distances… as well as a corresponding regular and recovery run pace.