A minimum “structured” plan would be rotating between days of short, intense sessions and longer, easier sessions, with some intervals. But it varies a lot depending on what your goals are.
Britishcycling has some ok training plans for free, if you want inspiration.
https://www.macmillan.org.uk/fundraise/charity-cycle-rides/training-plansIf you want to be more serious about it, follow what the other anon said and read Joe Friels The Cyclist Training Bible. That will give you a in-depth knowledge of how to set up a plan.
