For 80% of the time: ride long mostly at a comfortable but sufficient pace. Then, for about 20% of the time do intervals that make you want to vomit.
So, for 6 hours a week you'd ride about 4.5 hours a week at "endurance" pace while still paying attention to do the hills well.
Then, for 1.5 hours a week you'd do workouts that make you feel like you're going to die. Preferably split into two 45 minute rides.
You get nowhere faster on the bike doing roughly the same moderate thing every time you ride.