To stimulate adaptations, you have to increase training stress (and then rest enough to allow adaptations to happen).
The most effective way for most of us to add training stress is to add additional low intensity volume on top of the 2 higher intensity (SST on up) sessions each week.
Most will do better over the long term by increasing the length of that long ride each week, or stretching out some of the zone 2 days in between hard days, rather than trying to do more intervals.