Having done the 400k with you, I am sure you can do a 600, provided you throttle back a bit and pace yourself.
Comfort on the bike is vital, so more applications of chamois cream might be required. Think about any areas of discomfort, and try to improve them. Padded shorts, more padded gloves, warmer kit for the night riding, etc.
I am sure if I can do it, then you can.