Im a little late here, but this has been a big learning for me.  First I should say everyone is different.  You need to learn what type of sweater you are.
I dont know the exact numbers for other people, but I have found doing endurance training for anything more than 5 hours a week, I need less water and more electrolytes.  When I say electrolytes, I really mean salt.  Im learning that I am not a water sweater, I am a salt sweater.
People who say to drink more water, like 100oz a day, those people dont exercise much IMO.  When I would do that I would pee every 30 minutes and my urine would be completely clear.  I would get headaches in the afternoon and pains in my hamstrings in any run north of 8mi.  Once I started intense endurance training, that habit of drinking so much water killed me.  I am now spending a ton of time eating salt.  Like literally putting salt in everything.  I rarely drink water without adding salt anymore.  When I say salt, for me it means that 
fufu Sherpa Pink salt with all the minerals in it.  For endurance drink?  I love Gatorade endurance, I buy 
this one.  For carbs, I like 
this GU.
Now when I sweat, its more crusty and salty and my body feels so much better.  I can now feel when I need salt and when I have had to much.  Its like when I eat something salty it tastes like forever awesome.  Or if I have had too much salt, I feel thirsty and drink a couple sips of water.
The good by product of this is now I stop so much less on longer workouts to pee.  I drink so much less fluids before a race.  I also 
purchased these, but prolly wont try them out until the hotter months.
My A race this year will be my first HIM.  Im guessing ~6.5 hours of suffering.  What Im learning is that once you get past 3-4 hours of endurance, your body starts hating sugar.  For that ive been spending a lot of time learning to consume heavier calories during a race.  Ive learned along those times that apparently its a strength of mine as I seem to have an iron stomach.
PS - I love these nutrition topics!