The saddle time should build on itself to where it's not that big a jump. Try out a few 200ks first and see how you do. That's a great distance. Having someone to ride with can really make the time pass faster if you're compatible.

Riding at night: It can take some getting used to riding in the dark. I did my 600k with very little night riding experience. (I did the 600k with only a handful of double centuries under my belt and nothing longer than that.) Anticipating riding through the night was very stressful to me throughout the day of the 600k. It turned out to be not so bad, but I had some equipment failure issues and kinks I could have worked out beforehand if I had practiced riding at night.

When I did the full brevet series later on, I vowed to practice night riding. So I would get up at 3 or 4 am and do a ride in the dark around my house, in familiar safe territory. Helps you get used to new lighting equipment, etc. That really helped, especially with the transition to a Schmidt hub and light system.

Headlights can be tricky on longer rides. You will definitely want one that you can take off the helmet during the day because that weight is NOT something you want on your head for 30 hrs. I did my 600k with a Nite Rider Moab - it's like a flame thrower and puts the light where you look. However, for my 400 and 300ks I switched to a fork mounted E6/Schmidt hub system. I actually prefer that to the headlamp. I like the feeling that I am self-sustained and have unlimited battery (I carry spare bulbs). I do carry a LED light as a back up and of course (NON FLASHING) tail lights.

If you have other questions I'm happy to weigh in.