My first 24+ hour ride was a 600k that I decided to ride straight through several years ago. I didn't do any special training and wasn't in that great of shape, just decided to do it. I way underprepared for the weather and nearly froze to death in the last 200k. I wanted to take a nap, but I started shivering uncontrollably every time I stopped. It also didn't occur to me that everything would be closed in the middle of the night in southern Wisconsin so I ran out of food and water. When the sun came up I stopped at a grocery store for a few hours to eat, rest and warm up. Ended up finishing in 31h 22m which isn't that much faster than when I stop and sleep for the night. So there's an example of how not to do it. I've done a few since then with better results, but staying awake all night long is always hard for me. People who do it a lot tell me it gets easier with practice.
I agree with the suggestion of riding a 400k in 24h. It's so much easier to ride all night long with other people than solo.