Few starting points for farming credits in career mode seasons:
The races nearer to the bottom of the season page give more rewards than the first few races of the season. Also the races added in The Great Wall and San Diego Harbor updates might have slightly better race rewards than the season's original races.
To determine which races could be good candidates for farming:
The first step is to figure out the race times. Shortest tracks with the fastest cars available are natural candidates.
After figuring out a bunch of good quickly playable races, the race rewards could be compared and the farming calculations be done. Differences of tens of credits in the rewards for 1st place won't be that significant when playing for an hour (it's usually only a matter of weather one extra race is played or not, if the average race times are nearly the same), but when the differences in race rewards are counted in hundreds, the impact is felt within an hour of playing time (again, if the average race times are nearly the same in all options).
The final decision could be fine-tuned with personal preferences (liking a certain track, a car or a race mode). If two or three tracks are selected to farm credits, the play fatigue shouldn't be an issue.
Finishing in last position in any Elimination race will reward some credits (in ~45 seconds) without playing. This way the farming could be done while doing something else.
this (bit shorter than a medium length) track is playable in various different ways without too much penalty to the race time
Race 26 — The Dragon's Den — Classic — Class S: 1,840 Credits
an unrestrictive car selection in a medium length track with a couple of good bends (the curve to the right after a long straight before a tunnel and the final turns before the finish line, which is about the second best S-curve in the game IMHO after Monaco)
Race 27 — The Great Wall — Elimination — any Ferrari: 1,865 Credits / 840 Credits
3 min 45 sec race time when finishing first (might not be a good option if the race times of other candidates are sub 3 minutes), ~47 sec race time when finishing last (actually quite strong candidate. Elimination Race 24 only gives 161 Credits when finishing last).
Race 28 — The Rose of the Desert — Infected — Mercedes-Benz CLK GTR AMG: 1,735 Credits