My experience with hyponatremia comes from Ironman racing and training.  The thing I had to learn to do was to get enough salt into me.  Sports drinks have no where near enough, compared to the rate I sweat at.  I am at the high end of the range, 1.5L/hr or more.  (Measure dry before, go out for 10 hours, track fluid in, rarely end up peeing, weigh after, 1Kilo=1Litre).  

1L of sweat has on the order of 1-2 grams of salt. 

My issue turned out to be that during the swim, (I am slow, so 1:30 or so) I would sweat so much, but not notice since I was swimming that when I started the bike, I was already down on salt and it just got worse.  I started simply eating salt packets from a restaurant evey 1-2 hours.  Not too much at any one time, easy to get down, easy to come by, made a huge difference. 

But hyponatremia does really suck. I find it makes me fall asleep, even while riding, which is usually my first indication I screwed up.  Within moments of the salt touching my tongue, I start to feel better.  It is amazingly fast.