Tell us about your weekend ride

I don't even understand how anyone can spend more than maybe 8hrs on a bike. c.16 is madness.
Also how on earth do you eat so little during a ride?

All generalisations of physiology but (currently reading Faster by Dr Hutch)...

Slow enough, and with a well trained fat metabolism, and your body is powering you with mostly fat. The rate at which you're using carbs (fat burns in a carbohydrate flame) is low enough that the carbs can be replaced by eating (you can only take on a certain amount of carbs per hour). You can go pretty much forever at this pace.

Push a bit harder and your body needs more energy and this has to come from carbs (you're already at the limit of how fast you can process fat into energy). The carbs are being used faster than the body can take in new carbs so this can't go on forever, you'll eventually bonk but how long you can go for at this rate depends on the depletion rate. The average person has 1600-2000kcal of glycogen in the blood and liver - this is what you're depleting. A 200kcal an hour deficit will give you 8-10 hours before you go bang.

Push harder (up to and including race type pace) and you're way beyond what your body can take in and so the glycogen stores are depleted much more quickly, even if you ate like a horse. A 400-500kcal deficit will see you bonking in 4-5 hours; and sooner if you don't eat properly.

25kph average is the upper limit of my "all day" pace, it's closer to 20kph if it's hilly and I'm on fixed.

I can do 30kph average for a few hours, but I have to be careful what I eat. I've simply never averaged over 30kph for anything over that; mind you, I'm just an average fat bloke on a bike (with reasonable stamina and will/staying power).