Resting 80 - 100 kcals an hour doing nothing.
Rapid weight loss is fluid loss, like in the case I and OP has seen.
Last year I lost 4 kilos in a similar time but was on deuretics which I did not need, also BP dropping to 90 / 50 (deuretics) lethargy I put down to alergic reaction to the uneeded deuretic, once off the tablets all was normal.
You would have to include fluid in the food you consumed along with the tea breaks, as I do not frequent caf'es and stock up on fluid except a glass of water before leaving home, my fluid intake might well be similar to suposed no on the hoof drinkers
If you weighed yourself before leaving home and on return taking off fuid consumed on the hoof then for me that would be 0.5 litre (35 miles)but remember that you would continue to lose fluid at an accelerated rate after exercise untill you had recovered even if not out of breath.
Assuming that we would probably drink on return home we are all probably consuming the same fluid throughout the day, drinking on bike / on ride or not
Weigh yourself before bed and when you get up in the morning and you will find that 1 kg overnight would be quite normal
I think it would be hard to believe that any one doing say one hundred miles in a day would not consume more fluid than if they stayed at home, as you would loose more through increased breathing and sweat even in the winter, I do sometimes find I do not feel thirsty whilst cycling but I know I need to carry on drinking otherwise what would be the result at end of day
You can even without realising it dehydrate to the point where oral fluid intake WILL NOT rehydrate you at all, I have been there twice in my life and its not pleasant even though I never felt thirsty at all, so there was no signs for me and maybe if you are at a senior age it would be wiser to drink whilst exercising even if you are not thirsty, it wont kill you.
So what advise would you give to beginer cyclist
P.S. We have already had these conversations on previous posts