If you can comfortably ride 100mi in a reasonable time (7 hours?), then you should be able to do 200mi in a day (though don't expect it'll be as comfortable). Takes lights just in case.
If the furthest you've gone to date is 60mi, I'd work up to the 100mi and then reassess what you think is desirable/possible at that point. A 200mi ride a year from now should be easily do'able in any case.