if you can ride 100km without falling apart, you can easily do 160km. Work up to that distance first. And on any distance ride, don't think about the whole thing at once, just to the next food stop or some intermediate goal like that. Randonneurs commonly ride 600km in less than 40 hours. I don't think anyone thinks about that whole distance at once. I am usually pretty happy when I hit the 200km mark, generally trying not to think about the fact that I have to ride another 200km before I sleep and 400km before I'm done. I'd rather let that happy fact sneak up on me over time.