I've never passed out. There are a number of reasons for this. First, I drink enough sugar every day to more or less be considered a hummingbird by the Audubon Society. Second, I don't stand in one place long enough to let all the blood settle to my feet. I've heard people pass out in school choir all the time. Probably because it's so boring, or they're huffing in the back of the bleachers. Anyway, third, when I don't feel good, I stop doing what I am doing. Passing out is your bodies emergency system for when you need to lay down. If you don't lay down in time, it lays down for you.
Now, how can you tell you're not feeling good and you're about to pass out? Near as I can figure, it's the same as having a stroke. Everyone familiar with FAST? Face, Arms, Speech, Time. This applies to endurance racing to a T. I saw a lot of people passing out at Philly. Lots of great racers. Pros, hitting the pavement like a sack of potatoes. Firemen ferrying them away to little rubber rafts full of ice cubes (oh how I love ice baths). If only they knew how to tell they were getting FAST and not going fast.
FACE: your face is pouring buckets of sweat and snot all over your shirt. Then, suddenly, it stops.
ARMS: your pendulum like gait and arms waving around like a wacky waving inflatable arm flailing tube man suddenly lock up like Tin Man. Steam starts coming out your head. You poop a small dog.
SPEECH: people are passing you now. And they are talking to you. You can't understand them. The only thing you can say in response is "gulag."
TIME: you have been running way longer than you usually do. You've gone from track start to charity walk. It's time to stop, and sit down. Or your body will do it for you.
Sure, there's a lot of other components to avoiding the onset of FAST - hydration, salt intake, nutrition, sleeping in a pressure controlled hyperbolic chamber at altitude. But, when push comes to shove and you are right there in the heat of the race (literally, Philly was 104 degrees at 7 in the morning), you gotta read the signs. And either finish the damned race before they finish, or save some face and sit the you know what down.