I have problems with dehyrdation a lot because I start sweating quickly and A LOT. I can't really help you with longer races since I'm still in college, but I know what it feels like. I get a funny feeling in my jaw (almost like lock jaw), I get lightheaded, and my legs won't move. Happens to me probably 10 times a year. For me I have to make sure I'm drinking a lot of water with food in my stomach because otherwise the water goes right through me and I pee it out. So just because my pee is clear doesn't always mean i'm hydrated, it could just mean that my body didn't retain any of the water I just put in, so I like to have some food in there to soak it up, bread and bagels work nicely but so do a lot of things. That's the most I can help, good luck.