There is a strong connection between the speed people can run their 5 and 10k times and their and their marathon time. The quicker the times are the smaller distances (usually) the quicker the time at the marathon.
At RunYourPB we aim to work at helping you become quicker at the short distances, as this will give you the "spare capacity" to carry on for longer at longer distances. Imagine a runner who can run 30min for 5k and 2hrs for a 1/2 marathon. If they train and their 5k time comes down to 25min, then the effort required t run a 2hr 1/2 marathon will decrease. In practice they will either be able to run a quicker 1/2 marathon or or will be able to carry on for longer at the same speed. And that is what I mean by "spare cpacity".
When you run a marathon, for example, the first few miles(and even up to miles 16 or 18) should feel very easy and relaxed. We don`t (do we) run the 1st few miles quicker just because we can, as this doesn`t leave anything for the last few miles when we can really begin to run out of energy.