I'm no expert, I'm not even close. But I can offer my recent experiences.
I just finished the Davis Double, which for me was a lot farther than I'd ever gone. My max had been 112 two weeks before.
I bike commute, and that helps. I did a little weight work and core work, but not much.
I live in CA, and ride a lot on weekends too. Generally in the 50-60 mile range. As a run up I did 3 centuries in the month before. I also live in California where hills and mountains are ever present.
I got through my ride by trying to take it easy and not get excited, and drinking water and Hammer Perpetium like a mad man.
I'm turning 55 this year and I'm not now, nor have ever been an "athletic" person, but I am active.
This bike thing is do-able, and if you are working your commute and doing longer rides on teh weekends, well, 200K--why not?