Yeah I remember that now, so there's probably always at least one of each.
I had tremendous fun doing all of the enduros myself, even the Indy 500 which I had been dreading. I got the pace balance right after extensive testing and had to run as fast as I possibly could all race in order to make up for the occasional slide or wall-tap, resulting in a final stint duel with the by-then faster R8 all over my rear wing for the last several laps that I somehow managed to win.
The extensive testing is the key to enjoying these events, along with the patience. The more you know about what will happen and the more consistent you are with your pace, the closer to a toss-up you can make the race which makes it a lot more fun. Of course you can't be too afraid of losing, which is helped if you never use race mediums or softs, so you can put them on if you need to.
I'll stop now before wandering any farther off-topic. Haven't run any endurance races in a while except the GVS300.... Need to fire up rFactor....