Step 1: In The Long Run, It's More Than a Bike Ride
Consider the double century a long term goal. Train lots. Allow plenty of time to improve. Take up to three years, if you begin as a couch potato, There are training schedules to guide your progress. The schedules all have the same theme. That is increase your mileage slowly. Involve your commute to work in your training. Complete plenty of century rides beforehand. Try 2 century rides in 2 consecutive days. .
Recovery time after hard rides is important, as is time away from the bike, when you get bored,