You should try a hilly metric first, and if that goes ok, then do a hilly century, and if you can do that then do the 200k. A lot of the distance cycling takes time and experience, to learn what you can and cannot eat and drink during the ride, when you need to eat and drink, knowing what and when to eat and drink when a new route is more hilly than expected or a hot wind blows in your face for miles, contributing to potential dehydration, along with gaining experience on climbing, getting the leg muscles strong, learning how to react in certain situations so that the reactions are instinctive, etc.

If you train for a metric first, when you finish the metric you will know if you can do a century or a brevet. At the end of a metric you should feel like you can bike another 40 miles easily and with enjoyment. If you suffered during the ride, or if the next day you complain about torn leg muscles, aches, pains and extraordinary fatigue, then you know you are not ready for a century yet, or a brevet.

I finished a century yesterday with about 5000 feet of climbing. The temperature was about ten degrees hotter than expected. A strong headwind came up in the early afternoon. Some cyclists called it quits for mostly heat-related issues. When we got back to the starting point, one century cyclist was prostate on the floor shivering in spasms while event volunteers put ice on him and waited for the ambulance to show up; the man was suffering from heat exhaustion. These are the kinds of situations a cyclist has to train for to prevent anything happening to them on a long ride. It is best to train for a metric, and if the ride suits you, then train for the century, and build up to the brevet.