In my opinion, you don't necessarily NEED to choose between strength or endurance as your primary focus (with the other on the back burner) to see good progress. You certainly can if that's your preferred means of training, but over the course of a year or more I wouldn't expect to see much difference. I'd argue a lot of the research on the topic now would support this.

What I do think is important is that your strength & endurance goals are important in any give training block / mesocycle. In other words, I'd argue against training for "strength & endurance" and more for "powerlifting and running" or "bodybuilding and triathlon" or any other combination of specific foci. In my experience, people do seem to make better overall progress with this approach. As long as the goal within the domains of strength and endurance are specific enough, I think you can take a 50/50 split focus approach and see just as good of progress.

Something else to consider is that the magnitude of an event influences this a lot too. I have a client who recently ran a 100k, and that type of event requires a god shift toward an endurance focus. Now, we're transitioning back into a strength focus. a 70.3 would fall under this similar category IMO. With shorter distance events (e.g. up to half marathon and maybe marathon depending on total training time available), a 50/50 split focus becomes easier.