Each rank is successively harder.
I know I did mine a long time back at TUN, there was a circuit you could do in about an hour. You picked up and dropped off for missions on the route. Once you knew which stations to avoid it was pretty easy. No idea if it's still there, but I could do 3-4 percent of the last level to rear admiral a run. 25 hours or so ?
It made me question a system were they gave out rank for driving a delivery truck. So, like after 1000 delivery missions, they decide that you deserve a real engine of destruction, because dragging boxes all over the quadrant qualifies you for restricted equipment. I guess they totally de-funded their own version of the postal service and delivery truck drivers are heros.. I shudder to think what my local FedEX guy is qualified for.
Not like the Empire is much better. I mean, the Empire just wanted donations, which I can kind of see in a decadent, back stabbing, slave trading, monarchy where the only real crime is defaulting on your debts. If you're rich, you're in.