Zone 2 Endurance efforts with Sprints every 10min. You can choose to keep it in the same gear and spin it up to a high cadence or drop it to a harder gear to use it for time to get out of the saddle.
Alternetively you can just hammer for a 15s sprint and then recover in Z2 for a solid combination of workout.
Also consider varying your cadence in each block from anywhere from 60-110 rpm.