Not just intensity, but progression. If you start out doing 60s intervals at 400W (just as an example), it's not enough to just keep doing that same intensity week after week. You have to keep moving the goalposts. Don't wait weeks for your next FTP test. Target 410W on the next session. If you hit that, target 420W the next session. When your performance plateaus, add some variation. When performance drops below your baseline, it's time to program in some additional recovery.

Lot's of riders seem to want a training program where you just follow a fixed plan for N weeks. But sustained progress comes from the feedback loop of assessing your performance and constantly adjusting the plan. A good coach will do this for you. If you're going to self-coach, you will want to learn how to do it yourself.