Wow thats impressive.
I'm level 25 at the moment so I've only done the 300 Km (60 laps) Grand Valley endurance race. I did it in the Peugeot Le Mans car (cant remember name) while everyone else had volvos and the like haha. It took just under 2 hours and that was tough enough.
And I'll just take this opportunity to say that GT5 is my game of the generation and the best GT yet. I must be playing a different game to the reviewers.