Hi Jamie, one of the aspects that count a lot in a runner's favour, is experience.
Each year one should develop to a new level and 'muscle memory' plays a large part in being able to run at a higher level on 'little or no' training
avoiding injury and sickness and achieving consistency is what assists athletes the most.
over the years of training, the rule of thumb for easy runs in the early stages of training is to be able to run as easy as possible. In giving advice on the forums, the advice has always been to keep the pulse around 120 to 140 bpm, with the intention that when the athlete begins to train on the 10k programs (for example) their easy runs will be at a faster pace naturally but still the equivalent pulse rate.
Runners (athletes) unfortunately often do not have the patience to run easy (or at lower pulses) but instead continually want to know whether they rounding off into shape. Which can often be to their disadvantage.
Hopefully this is easily understandable, as often it is difficult to put things across via the forums