When I first started riding longer distances after a significant break, 30 miles was a long ride to me. Gradually I worked my way back up to 40, then 50 on up to 100 and 200 miles.
I can do 200 miles, but I still have the same feeling you do about people who can do 600k and 1200k rides. I'm pretty well bushed after 200-210 miles and have a tough time imagining myself doing another 200k+ on top of it.