I am stuck on the unmodified races too. I am at level 33 I believe though. The endurance race took me by surprise. I was winning the race even lapping some of them but did not pit, i figured I only have 4 laps left so why pit, because in Gran turismo the worse that happens is that your tires go bad and the handling is shitty but with Forza I actullay ran out of gas or my motor died because the car just stopped and would not move, I had to restart the race. I was pissed as hell.