500k is the ideal audax touring distance. Means you can do a 6am start, 300k, finish by 7pm, shower, few pints, meal out, loads of sleep, then a nice 200k the next day feeling super fresh.
The extra 100k in a 600k really makes it a different experience.
That's kind of the Liz Creese method of riding a 400. She liked 400s that started at 10am.
Good sleep in a bed before the start. 300km by midnight to 2am. Sleep about 4 hours or so then knock off the last 100. She rode a lot, often with a good bunch of riders and was faster than most.