Get Out Of Your Comfort Zone!!!
It's Gonna Hurt A Little
Training for fast short distances is not going to feel great, but it has great benefits for your aerobic capacity. Most endurance athletes rock the low intensity exercise settling into a pace and just cruising comfortably for hours at a time. However when training high speed work you can see big results in a relatively short amount of time.
Short distance fast pace running and the training it requires, not only can improve the flow of oxygenated blood to muscle tissue, and help improve leg speed and efficiency.
It Won't Disappear
Adding in high aerobic workouts for a short distance can only benefit your long distance training by putting your body in a high lactate state, and allowing it to recover will teach your body to mitigate larger amounts of lactate over time. This benefits your long easy run by helping the body to more efficiently handle smaller amounts of lactate produced at low aerobic intensities.
But What Does It All Mean
To sum it up training for a fast 5km will help you not only increase your baseline running pace, these types of events can fit anywhere in your training for your goal race. You may find a big benefit in doing this work early and using these races as early training for building, or even using them to test your fitness in later stages of your goal race training
This type of training should be done slowly, building your speed time at a high heart rate over time. Keeping to the 10% rule especially for new runners who are who are not used to this type of stresses on the body. You can easily get injured if you just go out and blast this type of training off without the build.