The game is as challenging as you want it to be. Just because you can doesn't mean you have to fully tune a car to win by miles. Look at the suggested cars before starting the race and pick the slowest car or pick the top car but put Comfort Hards on.
Also it seems you're judging the game by the trophy list. If you play the game solely by aiming for trophies it will be boring and an endless grind. If you play to challenge yourself without aiming for trophies, you'll enjoy it more and them trophies will come easier and quicker because you'll be playing the game more.
Imo, GT5 is the best racing game currently on the PS3