The speed that is best to drive is the speed limit or 5 miles under the speed limit.
Driving too fast can be dangerous and driving too slow can also be dangerous.
If the speed limit is 55 mph to 65 mph then it's best to drive those speeds and keep up with traffic.
To drive fast on back roads you do it the same way you would drive fast on other roads.
However you would need to watch more closely for any vehicles that may be coming out of driveways, yards etc.
And also watch out for any possible curves or vehicles coming around the corners.
When on back roads you should drive a bit slower than you normally would on other roads and especially if it's on a dirt road.
Dirt roads can get sandy and cause you to lose control of the vehicle so keep both hands on the steering wheel.