Increase your training frequency, then the scope, and then the intensity
"Most of the problems athletes encounter is due to the fact that they violate this golden rule. It really is quite simple. Those who want to optimise their training must increase the frequency of their training first and foremost (4 times a week instead of 3). If you cannot train more often, then extend the scope (run 5-10% longer). Only once you have reached your maximum frequency and scope should you increase the intensity (run 5-10 seconds faster per kilometre)."