I've only done one but it went well and ended up just being a century that took twice as long. I mean I did the same things I do on a century just until I hit two hundred miles. Ate, drank, sang songs and pedaled along - it was hard and I was glad to be done but like I said it went welll. Prior to the ride my longest recent ride was a 200k about a month before, my longest ever ride was 150 miles the summer before - I didn't do any special training. Just the 200k and my regular riding of up to four hours a few times a month.
The only thing I did different than a normal century was stretch several times an hour from the beginning. I can brute force a century riding with poor posture and pedaling squares but I knew that wouldn't work for 12-16 hours so I made sure that every opportunity I was stretching on the bike while coasting down a hill or fast flat section. Paid more attention to posture and position and made sure to move around hands often.
The ride also started and ended with about a half hour of darkness, I didn't do any prep and didn't need seem to need any for alertness but an additional 35 miles of riding in the dark and I most likely would have added a early evening coffee or something. Transition from day/night after a full day of riding sometimes hits me hard so I prepare now packing caffeine tabs and extra clothes.
I think if you can do a sub-6 hour century you're in pretty good shape, personally I'd bank on just doing the same stuff with the understanding it's an additional hundred miles and there may be boredom/adversity compared to a regular century. I had an MP3 player that was a godsend from mile 170 until the finish.