The foundation of any solid running training plan is aptly named Base Building. You should build your based by running at an Easy pace, or E pace. The majority of your weekly running, even up to 80% of the total weekly distance, and your warmup and cooldown for workouts should be at this pace. You should be able to easily hold a conversation with someone while running at your E pace. You could probably even complete this pace while only breathing through your nose.
This pace is likely even slower than you think it should be. The benefit of running at this intensity is your body will make physical changes. Your body will create additional capillaries to the muscles. Capillaries are tiny blood vessels that bring blood to and from the muscle cells. The blood is carrying oxygen, so the more capillaries you have, the more oxygen you can get to the muscle cells.
Your body will also create more mitochondria in the cells. The mitochondria are the power plants of the muscle. This is the part that takes the oxygen and nutrients (glucose/sugar) from the blood and burns it in the cells to let the muscle cells do their job. If there are more power plants, then the muscles can work faster, longer, stronger, and more efficiently. Easy running also increases your body's tolerance to the demands of running by strengthening your tendons, ligaments, and muscles. All of these adaptations contribute to improved running capacity.
There is no need to go any faster than an easy pace to yield the benefits discussed above. In fact, if you go too fast you'll interfere with the desired adaptations. Both beginning runners and veteran runners should spend the majority of their running miles at this easy, "base-building" pace. Only when this base is built (approximately 6 weeks of easy running, for both beginner runners and veterans within a specific training cycle) should faster runs be incorporated.
Use the table below to see what your E pace should be according to your VDOT value:
For some of you, these paces may seem really slow compared to how you usually run. Do an experiment and see if you can slow down the easy runs. You may be surprised that when you truly keep your easy runs easy, and save the hard work for the workouts, your running times will improve. It's a process ;-)