It depends on the plan and level, but overall I think the most efficient method would be to add more volume to existing scheduled cycling workouts. Increasing the frequency of cycling workouts would be just fine too, but the effect would be virtually the same as adding time to scheduled sessions, and this method has the advantage of not having to add an entire additional workout (and the associated additional time to prep, shower, etc…)
Which sessions to add to? I would add to the longest ride first: make the longest ride longer, then add time to the second, then third longest. This provides maximum benefit to the rides where endurance is the primary goal of the workout.
Ideally, you would also adjust the intensity balance for the week, adding 12 minutes of moderate to high intensity for every hour of cycling you add to the week (20%). Remember, you can add this additional intensity to the interval workouts. For example, adding two 6-minute Zone 3 intervals to an existing interval session covers an additional hour of cycling for the week and maintains the 80/20 ratios. It’s ok if one workout is 50/50 balanced as long as the week (or two weeks or three weeks) works out to 80/20.