Your PLs increase according to the PL of the workout you you complete. So, if you do a 4.5 workout, your new PL is 4.5. You won’t end up with SS power in your endurance workouts. Eventually you just have to make them longer to increase the PL.
But, there is much debate over the PLs for endurance workouts. See the following thread:
My suggestion would be to keep using alternates to select easier endurance rides.