I learnt something...I sent my Bob to win the Japanese car event in extreme...and despite having a superior car (787b stealth)...he led until the penultimate lap, where he got overtaken, and finished 6th

.
I thought perhaps he was so tired, he practically died at the wheel...I tried it again, keeping an eye on proceedings. I think it must have just been the tires that became unmanageable...and he was in the rain, so he had to be uber careful and consequently lost...because this time, I watched him, and he was winning easily (not in the rain).
As with last time, he lost all physical and mental strength by about lap 13/30...but his performance only dropped when I let his bar stay cool. He tried to pit at about lap 26..I said no. He proceeded to spin out 3 times that following lap and lose half a lap's lead. So naturally I let him pit the next time and he won with his new tyres.
So when it came down to it...despite having no engery physically or mentally for more than half the race, his performance only really dropped when the tyres were shot...and he tried to rectify that himself...so I think as long as your driver is able to respond when the pack innevitably catches him after going too slow for too long and fight again, he would have no problem doing an entire race himself.
However, I'm training a second racer who's gaining levels quickly.