Several ways to raise your upper threshold / FTP, starting with longer intervals to target 40km / ~25mile TTs. FWIW I was in high school when you were born, so fighting age related declines and starting my eighth year of cycling.
At a high level, versus 3-5 hours/week, I’ve seen the most fitness improvements from increasing average volume to 7-8 hours/week largely irrespective of intensity:
In WKO I’m tracking more fitness metrics, that is a simple view.