I did Vätternrundan one year on a fixed (front brake required) and I thought I was going to die. It is 300 kilometers (184 US miles) and claims to be the longest/largest (15,000 participants) single day bike event in the world with some 34 countries participating (by the way, it's in Sweden).

More info Vätternrundan

It almost killed me but the funny thing is I am considering doing it again...