If it's your first time driving on a particular stretch of road, you should keep your speed in the 90-95km/h range. Let cars pass you if you see a lot of cars behind you.
Some of the drivers on the country roads drive those routes regularly, so obviously they will be more comfortable travelling at a higher speed - you don't need to feel pressured to travel that fast.
With regards to the speed tolerance - my advice is to follow your speedometer and travel at the speed limit.