Arcade, custom race, either of the 2 ovals, endurance race set to how many hours you can spare, boost off, pro difficulty.
I was using a Miata (with full weight reduction & power upgrades to n199) against a single grp1 opponent. This worked well for a while, but then the AI kept punting the Miata, knocking it off track and causing the game to crash after a while. I switched to a single grp3 opponent, and this worked fine at a marginally lower payout.
Others report still being able to run Miata against grp1... I think the payout for n100 v grp1 over a 24h race is 38k exp points (plus credits & mileage)
Set the race up, start the race, then steer the Miata against the outside wall and let it run.
Your controller needs to look something like this...
Steering to the right, gas full on.
Best to check it periodically if you can to make sure it's still running correctly.