I'm trying to do the same thing. Unfortunately the sad fact is the AI isn't really that good (at least not on A-Spec). This is compounded when you factor in events with variable weather, because the AI drive ultra-conservatively.
The first thing I would do is make sure you disable SRF, then use tires that are at least two grades less than the AI. In addition, go ahead and buy every single tire type for the car you're using. This way, if you find it's still too easy, you can put on even worse tires on your next pit stop.
I'm currently running Suzuka 1000km and using tires two grades lower than the AI as stated above. I'm still faster than the AI, but, my tires are running out much quicker, so I'm pitting more often - this was keeping things interesting, that is, until it started raining. Tire grip as it relates to a wet track definitely doesn't work as intended, as I was able to keep running on slicks even when the track was 100% wet and I began lapping 5-10 seconds faster than the AI, which completely blew up the competitive nature of the race. Not only that, but the wet track makes the slick tires last forever (I pit for fuel after 28 laps or so and my tire wear was around 50%)
I suppose I should artificially pit and put on wets to keep things closer, but once you keep handicapping yourself to keep things interesting, it just doesn't feel as 'fun' anymore.