I think that's something that most beginner runners don't think of. Endurance base is so important but also the one that is neglected a lot of the time.

It's easy to think "I want to run fast so I have to train fast" but it's harder to get into a mindset of "I want to run fast so I better not tire out as quickly". Long steady runs are the best way to build this up (long being relative to your level, of course)