What helped me improve my speed was listening to the Peter Attia podcast episode with Iñigo San Millán. He talks about how he trains pro-level cyclists with heart rate zones, and how it relates to mitochondrial function -- and what heart rate zones to train at to maximize the mitochondrial efficiency, so you do not switch to glucose prematurely.
The regimen he recommends is 80% of maximum heart rate (around 160 for me) for a few hours a week. This is actually a pretty relaxed pace where you can still talk but are breathing somewhat heavily. Looking at the training schedule of elite level cyclists, apparently they do this most of the year and do little high-intensity to prep for a race (in terms of percentage training time).