There is some blueprint tracks that goes up and down the highway. Get e.g a Koenigsegg Agera RS or another bloody fast car. Put on some chill music. Do a 30-40min event, and you'll get some hundred thousand credits, and quite a few wheelspins to go as well.
I know this may be frowned upon by some but i found this and got the maximum amount of credits possible, i love my racing setup but i need the cars to get the most out of it and im awful at paint jobs and those 3 hour races are pretty painful with weak cars lol https://www.ebay.co.uk/itm/292843398442
The game is much more fun now