Nothing magic to do. Just ride increasing distances! If you enjoy riding and have a decent bike of whatever flavor you prefer (hybrid, road or better yet, recumbent), that's easy to do.
Sometimes reading the advice given, and it's all good advice by the way, I think we try to make things overly scientific and complicated.
Just get out and ride as much as possible, be healthy as you can and you'll find it reasonably easy! Your first century is more of a psychological than a physical hurdle! If you enjoy riding I'd bet that after the first century or two you'll be passing the 100 miles mark and asking yourself "do I have to quit now?"