I see you're from the UK, so you naturally drive on the left side of the road already, so you are prepared for that.

I find driving in Thailand is not that bad at all. The lack of using the horn as a weapon except for emergencies and the philosophy to "give way" when driving is actually pretty good compared to other places where driving is more of a competitive sport. And for the most part, the roads are generally well-maintained with exceptions for area where they are building new expressways and new transit lines. Unfortunately there's plenty of drunk drivers on the road especially around holidays and there's no shortage of traffic. And be careful of those u-turns on the highways.

The drive between Bangkok and Khao Yai is not bad especially if you avoid the holiday/weekend traffic going to/from Isaan. And you really do want to have a car at your disposal in and around Khao Yai.

-David