i think the first and the last class you do are the hardest, as the first seems to take forever (especially D, as those cars are the slowest). And by the last class your anxious to finish. Actually i believe that Career may be a much bigger chore, though i don't spend nearly as much time with the races as with hot lapping.
All my times for Career are made in the first time, though sometimes i'll rerun a race cuz i messed up.