Completing a double century or 200 mile bike ride, in one day, is a great accomplishment for any recreational cyclist. It is also a makes a great story when you get old. There are plenty of organized rides all over North America. Most have a small entry fee. The support that is offered varies greatly from ride to ride.
Step 1: In the Long Run, It's More Than a Bike Ride
If I can offer you one tip it is wear sunscreen. If I can offer you a second, it is loose weight. Extra weight slows you down plenty. Calorie counting it the most brutal as well as the most successful way to loose weight. Depending on your circumstances you can eat less or exercise more. Keep a diary of how many calories you consume. There is no need to be accurate. You can round off items to the nearest 50 or 100 calories
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,