Ride a number of 200km rides. Maybe 10. Then try a 300km ride. You won't feel ready for it, but it will be doable. Then you should be ready. You aren't listing a distance so I"m just guessing how far you want to go. I would aim for 400k because I'm not an overachiever. Riding all night is something you can't really practice for, but you can get used to it. Eastern PA randonneurs had an overnight 200k, which was fun even though I might have grumbled about it a little. But I didn't really think it was good for anything. OTOH, I have a lot of practice riding all night. Just a note on route design. A properly designed 1200k is one 100km stretch after another. That's a decent interval to ride without major stops. Shorter rides and rides designed by inexperienced route designers often have more frequent stops. But you can go a long way with planned stops every 100km. And recognize that a lot of stores close fairly early nowadays, so you have to plan accordingly.
I don't ever carry a blanket, but you have to carry clothes to match the weather conditions. It gets cold at night in most of the U.S., even in summer. It's worth figuring out how you are going to do this and take the clothing on shorter rides to make sure your plans are workable.