In my admittedly limited experience, a 12 hr race can feel a whole lot worse than even a 600K. In other words, the peak of discomfort during the 12hr race (where you might ride, say, 200 miles or so) is much worse than the most discomfort you'll have at any point on the 600K. Even the hardest brevet is ridden at an overall lower pace / level of intensity than something that is actually a race. Its a race, after all. You should finish the race devistated. I haven't finished any brevets feeling absolute devistated - like having trouble standing up or walking. I felt that at a 12 hr race. I don't really want to think about a 24hr. Yikes.