Ride 7-10 minute intervals as hard as possible once a week to increase your anaerobic threshold and power for durations from minutes to a couple hours. It's simpler to do those up-hill because you won't need to pedal significantly faster accelerating down small less than flat spots. Some people also find it easier psychologically to push themselves up-hill; although you want to do that at a high enough cadence you're not limiting yourself.

Spend the rest of your time below your aerobic threshold (where breathing becomes rhythmic, conversation doesn't flow, and lactate/hydrogen ions accumulate) to improve power for durations over an hour.

Have a lower volume, lower intensity week out of every 3-4 to allow adaptation to occur.