For overall AI difficulty after championship has started, go to the CCH file and look for the quick AI difficulty setting. There could be more than one season listed so carefully Scroll down the file till you see your championship.
You cannot leave the game to adjust difficulty between qualifying and race, as the game has a bug, it will not save your qualifying position if you leave the game after qualifying. If anyone knows a way round this it would be good to know.
BUT you can do your practice sessions, and then leave the game to edit difficulty.
To edit qualifying and race pace separately - for example, if you want qualifying to be more difficult than the race, go to the track aiw file. Look for the lines
WorstAdjust=(0.800)
MidAdjust=(1.000)
BestAdjust=(1.200)
Qualratio=(1.05)
Raceratio=(1.00)
You can edit the worst mid best figures, or just the qual and race ones.
In the above example, the cars should be quicker than usual in quaifying, but normal pace in the race.
You can adjust session times in the track gdb file.
Heres an example, you can add, edit or remove anyof these lines:
// SESSION TIMES AND DURATIONS
TestDayDay = Thursday
TestDayStart = 8:30
Practice1Day = Friday
Practice1Start = 11:00
Practice1Duration = 60
Practice2Day = Friday
Practice2Start = 13:00
Practice2Duration = 60
Practice3Day = Saturday
Practice3Start = 9:00
Practice3Duration = 60
Practice4Day = Saturday
Practice4Start = 10:15
Practice4Duration = 60
QualifyDay = Saturday
QualifyStart = 13:00
QualifyDuration = 30
QualifyLaps = 12
WarmupDay = Sunday
WarmupStart = 9:30
WarmupDuration = 60
RaceDay = Sunday
RaceStart = 14:00
RaceLaps = 50
RaceTime = 120