Percentage of Threshold Pace

When training for long distances runners rely on paces. But interval training sessions require different percentages of a threshold pace. Say I run a 13:36 min. mile (my threshold which is timed=run as fast as possible for 30 min.) I’ll need to run at 80%, 75%, 50-55%, and 40-45%, during Easy Training Runs. To set an interval timer runners need to calculate these slower paces in advance of a session. These times (paces) should be larger numbers as they are slower times. These slow/easy runs build endurance and allow muscles to heal/adapt for speed runs on another training day. Those speed intervals would be faster times calculated after a session has been performed, not prior to the session. What we need isn’t just a normal pace calculator, but one that shows what paces to run that are slower and at a specific percentage of a given pace.

Also, runners use either min/mi. or min/km and, so imperial-metric conversions are necessary.
Thank you for considering this request.