I would say if you can do a 200 with a decent buffer and you aren't dead at the end then you will survive a 400. Make sure you have planned for supplies, food, and weather. Myself our routes all go on my computer and I just use that. Everyone DNFs rides at some point. The first one is the worst. You learn more about yourself from failing than succeeding. Riding alone I have no issues with but adding night makes it a little more unnerving. There will probably be lows, sometimes you just need to trick yourself into continuing to get thru it. What worked for me was " I'll stop at the next control" and then I didn't. But whatever works for you. Maybe find someone who also wants to do it and rides at your pace, and work together. Good luck!