My problem with 600k's has always been stomach issues related to hydration and electrolyte balance. The problem is that we usually do the 600k at a time it's likely to be hot, but early enough in the spring that I'm not yet acclimated to heat. It seems like later in the season I don't have any problems with that. I haven't had a strategy that kept me far enough away from hyponatrimea, and I end up taking electrolyte pills as an afterthought. I am getting better at dealing with it though.
I not really confident that I can always manage this problem on every ride, so I purposefully get into a fat adapted state if I can. That makes me slow sometimes, but I finish. I have been riding my trainer for 30 minutes before I eat breakfast a couple of times a week, that's supposed to help. On my first 200k this year I barely ate anything, that was fun, but it did help on later rides.