Our overall goal with training is to increase pace while working at the same effort. There are two ways to do this. One is to train the top ends of the scale and extend the pace we can run in Zones 3, 4 and 5. This type of hard effort is what we see the majority of non-heart rate trained athletes executing. They run extremely hard in the hope of running at a certain hard pace will eventually become easier to them.
First, if you are constantly running hard or very hard, you will see minimal to no gains in your overall total endurance for an event such as a marathon or ultramarathon because you need the easy work to provide the training stimulus.
Second, running at such a high intensity day-by-day causes the body to break down and injury is a concern, especially in new runners.
Third, from a mental standpoint it is tough to find success in performing at a constant high output.
Take a look at this video where I go into more depth on this topic and be sure to let me know what you think!