Iirc, foremilk / hindmilk is like turning on a hot tap, as if the foremilk were the cold water & the hindmilk the hot. As the hot tap runs, it starts off cold & gradually gets warmer. There isn't some invisible "switch" where it suddenly turns into hindmilk.
Pukkapatch I wasn't saying you don't need to drink water, but I have heard it said so often that you need to drink lots & lots & lots of water to increase or even maintain your supply. Breastfeeding can make you thirsty just like any activity, but it is a myth that you must drink loads more fluids than normal.
As for the losing weight, the way I've heard it described is that during pregnancy your body stores extra fat specifically for the purpose of making milk. So that if you don't bf, you have to work extra hard in order to burn off those additional fat deposits and get back to your pre-pg weight.
As for burning 500 extra calories I've heard it put at 250 calories, 100 calories and 500 calories on different occasions. I wonder if it's more accurate to say it burns up to 500 calories a day?
I think as with all these things, some of these myths may have a small basis in fact.
So, for example, "'if breastfeeding is hurting you're not doing it right' is a myth" might well have basis in fact, in that if breastfeeding is hurting a lot after the first few days, baby might still be getting enough milk (i.e. you're not necessarily doing it "wrong") but a few subtle changes in positioning and attachment might decrease the pain drastically.