Question for all the knowledgeable multisport athletes.

I am going to be competing in 2 duathlons and one mini triathlon this upcoming spring/summer. The most running I will be doing in either of those races is two 4km runs during one of the duathlons.

So my question is, when it comes to training, does it make sense to train for longer than a 5km run? I've done one 10k training run, was that a waste of time??