I apologize if this has been said, but the right speed is that which gets you there comfortably. Whatever average speed you achieve is fine if it meets that goal. You could probably ride faster, but you'll arrive sweated up, and probably continue to sweat for a while when you arrive.

That, the mos efficient way to ride is similar to the rules for saving gas in a car. Be patient and accelerate slowly except for initially starting from a stop. Climb hills steadily trying to carry your momentum to the top. On longer hills settle into a comfortable sustainable gear and work it so you're not shot when you reach the top.

Also, try to ride as much of the time at or near your average speed. The wind resistance is proportional to the square of speed, so riding faster has a big penalty in energy needed. Someone who does half the trip at 10mph and the other half at 20mph, works much harder overall than someone doing the whole trip at 15mph.

So use hills to advantage, especially long shallow grades, but otherwise work on getting your slow speed faster, rather than trying to see how fast you can go for short stretches.