I had to make some decisions about how much to include from FASA. I didn't like "Xenophon" as a name for Garth's ship. The Marklin Class was just a destroyer and I think Garth would have had a true ship of the line. So I changed what FASA came up with, but I give it a nod. Thus the war with the Klingons which has lasted four years.
Ultimately I am not following FASA, but take inspiration from it. Just as FASA took inspiration from "The Final Reflection" by John Ford. Let's face it, that book had a big impact on many of us.
There is a new ebook fan fiction called "The Four Years War" by Stephen Fender. You can find it here
He does an admirable job taking FASA and putting it in novel form. But Garth's role is minimal, and this was my major problem with the storyline from FASA.
Garth was the role model for Kirk and a whole generation of Starfleet Captains. And while Canon doesn't have The Four Years War in it, we know that Axanar was a major victory that somehow saved the Federation.
So in "Axanar" Garth is the Patton of his day. He is THE guy Stafleet goes to when they plan the major engagement of the war. And that is because he has been kicking ass for four years. He is promoted to Fleet Captain and given command for a battle the Federation hopes will end the war.