There is no answer to your question. Or rather, it has already been given:
"Cycling never gets easier, you just go faster."
As your fitness improves, you have two options:
- Ride faster but harder, or
- Ride the same speed but easier.
Of course, if you ride the same speed, you will stop improving.
Besides, everybody's different and so there is no correct answer for everybody. if there were, everybody would be doing it.