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I struggled to eat properly (and still do sometimes but am much better now).
Having benchmarks helped me. Doing 40 miles? Have a snack at 20 miles, something with 20-40 grams of carbs. One bottle should be done halfway, the other by the end of the ride. Anything longer than 60 miles eat something at least every hour. Whatever time you first eat something (9:45am) make sure you've eaten something by 10:45, earlier if your moving fast / working hard.
For audax length do the above plus stop at mealtimes and have actual food, sandwich / burger etc etc. Use hydration tablets if you suck at drinking, will give you some leeway. That's all I've learned tbh. If I follow that I feel fine all the way round. What fellow good audaxes slammed into me was being disciplined about eating and drinking was at least as important as the legs.
Tldr do what hippy said and get good at eating and the distance will follow way easier
600km, on road, 40hr limit, hilly and potentially 28-30 degrees. I want to do 24 hrs before thinking about a wee kip if necessary, that would be cool.
I think the thing I'm worst at is fuelling and hydration. Now obviously we are all different but I struggle to take the time to eat properly on rides of any length tbh.