Running hard on every run will result in an injury and no improvement to your aerobic ability. You have to run slow in order to become a fast runner. I hate it when people say you need to work harder to become a better runner. Hard work is how you ensure you remain slow.
So true, David45. Running is also a sport that's uniquely attractive to whiny failures. Team sports weed out the pathetic losers early on, but running allows them to remain on the periphery indefinitely, cowering like a whipped dog and lamenting their poor fortune. I'm sure your future employers will be thrilled by your attitude.