Barry says: The “drink 2 litres of water per day” is fallacious. (The original recommendation was 2 litres per day including all the water included in food). The current advice is to drink sufficient to satisfy you thirst. However, carry plenty with you when strenuous walking in hot weather. You also need some salt to ward off cramps. I carry salted crisps and sometimes put a small pinch of salt and sugar in my water or 50/50 pure fruit juice which is just as isotonic as proprietary sports drinks.
(pic by Muffet)