@Don Wakita the race you choose is a good one but......but need one hour.
As they wrote above less " stressful " races are Sartegna and La Sarthe.
At La Sarthe i use to do the majority of my grinding races and now i " test " many other cars since the 700pp limit allow many of them to participate.
The same applies for Tokyo( variety of cars ) but i just " hate " it and its quite common for a player that isn't use to it to loose CRB.
A big help for griding is also the weekly races especially when rewards are only credits.
Also you can try to participate at Time trials .
A silver score for example which isn't that hard ( most of the times ) can give you 1 million plus per week which can be very helpful to griding procedure plus you deal with something different.
Most important - at least for me - is to have a plan for credits you earn and where you spend them.
For example:
Isn't wise at all to spend credits on swaps at early stages.
The same applies if you're buying the unicorns of the game that cost from 10 millions and above and being " broke " just to get one of them.
Also a good tip that i use personally was this one.
I decide to have an amount of credits that i never touch unless was needed for a certain race or fill a cafe menu or got an invitation ( especially for that ) for example.
The amount of credits that are always in the bank ( short of) is something that you will decide what will be according to your needs , liking or whatever.
Mine was 30 millions but that's what i choose.
My last advice : NEVER,EVER leave an invitation to pass by .
Always get invitation cars when you get one and if credits permit buy MORE than one .
All cars of the game will made a circle sooner or later, but with invitation cars you never know when you will get one .