While you can set difficulty in, say, a race in the event list, you cannot select it in World Tour. Apparently as you level up within the mode, the difficulty will increase (how far up, I'm not sure. I read one review where the user said things started to get spicy for him when he hit around level 32).
In addition to difficulty via level, the game is also supposed to be calculating difficulty according to your driving skills within that particular level. So while you may be smoking them now no matter what, the increased levels and 'modified' AI based on your skills should eventually get you where you want to be in world tour.