I'm sure we all know the answer to that - which is , one spoonful at a time!!
Not long ago, I read somewhere that to complete a huge unimaginable task, it is easier to break it up into smaller tasks. This article was written within the context of running - and argued that before we can run (say) 10K continuously, we need to be able to run it non-continuously first. I am now applying this principle to my HM training. It is just unfortunate that HM and M training takes so long time for each event - it is difficult to practice. 10K is however a good distance to practice this principle.