I don't think optimizing pace is the right approach. Pace matters for races and speed sessions. For many other runs, especially easy and recovery, running "by feel" is also important. Usually it does correspond to some pace, but it means no looking at your watch and worrying about the pace.For me my recover runs can range from 7.30 min/mile to 9 min. Also I try to stay in in my HR Zone 2 for many of my runs. I'm currently targeting a 2.40 marathon in a few weeks