Not "when you are ready". Set a goal date or else it will always be a "someday" type of thing.

If you are riding 30 miles in a single ride now, you could set a goal of riding the 200k say next July. Figure out how much time you have left to ride this season on the road, maybe into October? Plenty of time to work yourself up to a metric century.

Then figure out how you're going to keep in cycling shape over the winter, spin class or learn to love the trainer with some road riding when you can.

Then plan on getting on the road whenever you can when your cycling season starts, late March? Start over again at 40 miles and work your way up to 55 by the end of April, then 75 by the end of May, then 100 miles by the end of June.

Over the winter, figure out everything you need to about your proposed route. What roads to take, where you can stop for restrooms, water, food. What you need to carry. Test your equipment all spring.

Make a plan, know what you want and you will get there.