I can’t remember the source, but I convert RPE to pace thusly.I also can’t be sure if “10k” is your PB pace. Happy to be corrected.
As others have said, it’s not strict, it’s a guideline. If it feels too hard, then it is!
RPE 2 = Recovery Pace = Light Jog/Brisk Walk = <55%
FTP RPE 4 = Easy Pace = Marathon = Endurance/55-75% FTP
RPE 6 = Easy-Moderate Pace = Half-Marathon = Tempo 75-85% FTP
RPE 7 = Moderate Pace = 10k = Sweet Spot/85-95%
FTP RPE 8 = Moderate-Hard Pace = 5k = Threshold/95-105% FTP
RPE 9 = Hard Pace = 400’s = VO2 Max/105-120% FTP
RPE 10 = All-Out Pace = 100’s = Anaerobic/>120% FTP