Apparently the AI are tougher on 100% distance, simply because they aren't effected by race fuel, like we are. And our fuel simulation isn't relative, unlike in rFactor where the fuel loads decreases relative to the race distance, so the cars are actually easier to handle here in a shorter race.
Until I knew that I was doing short races, because I don't always have time, but I'll have to start doing longer races. I've never done 100% before...