Like Famine mentioned, pay attention to the weather and change tires when needed.
If you want it to be more challenging, try using a GT3 car or something that is around 700-750pp. If you want a easy race, use the Bugatti VGT GR1 or Mazda LM55 Gr1. If you are trying to get fast laps, I'm loving the Porsche 962C with Praiano's 800pp tune... I'm trying to get a 2:03 lap with it but struggling to get below my current best of 2:06:300