Races in prologue are not taking into account if you set professional physics and that's pretty bad system. In many of them gold is simply untouchable thing. You'll see in A and S class races, it's just bloody hunt even in beginner physics mode.
Whole game system, times to win and races overall are set this time again pretty synthetic, it has nothing to do with average player skill. It's the same thing that was very annoying in GT4, because fair, balanced and entertaining races were very hard to find, it was even straightforward win or be doomed to lose.
Every single race must be tested by many players and if not, GT5 will be just another car show and no good game, no fun. In GT4 I reached 67% and from that point, nothing made me progress further. Every time I ran the game, I selected RUF CTR Yellowbird and beated Nürburgring till night.