I do a long ride each year and it is about 12 hours long. In that 12 hours- I drink 12 litres of water. That may be a lot but I do not suffer from de-hydration. I also do not pee a great deal either- I sweat the moisture out so my theory is that To stay hydrated under strenuous exercise, you have to drink a lot.
Incidentally- Prostate cancer is a killer. And that is from someone who was finally declared clear of it 5 years after surgery to get rid of it.
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How long was I in the army? Five foot seven.
Spike Milligan