Trust the plan and follow the plan
Because I'm self coached, I don't have the option of having a plan tailored specifically to me. There are a lot of plans out there, and I've chosen one that I think will work well. It's a fairly aggressive plan, including 3 each of the swim/bike/run plus a brick every week. From this book:
Now, despite that, my initial reaction was to add extra run mileage. Because, somehow in the last year and a half of getting involved in triathlon, I've amassed more knowledge then Matt Fitzgerald. I mean, sure, he wrote an okay plan, but I know more then him so I'll just tweak it...
Last year, my biking suffered because I prioritized the running. Now, I'm actually okay with the results of that. I couldn't run before starting this, and I could bike. So, a bit of imbalance was okay for a while. Not anymore. In fact, I think one of my other goals for the coming season is going to be related to getting stronger on my bike. The bike is a very important part of the triathlon.
So, with that in mind, I've made the decision not to do a spring half marathon. I already know that I can do the distance, and a stand alone half marathon is not really the same thing as the run in a HIM anyways. It does not fit into the plan, and if I really want to prioritize my half iron, I have to keep that in mind when choosing other races.
The only alterations I'll be making to the plan are to make it fit with my schedule (ie: changing the rest day, since I have coached swim on Mondays, doing a tune up race on a different weekend because of when it's available). Obviously, if life throws me a curve ball like illness or injury, I'll also have to adjust things. But, what I won't do is change things because I think I know more then the person that wrote the plan.