I would never run faster then the estimated MAFF paces predicted by maffetone from different runners.
8 m 00 mile pace for 6.30 mile pace on 3 mile run
7 m 30 mile pace for 6.00 mile pace on 3 mile run
7 m 00 mile pace for 5.30 mile pace on 3 mile run
6 m 30 mile pace for 5.15 mile pace on 3 mile run
6 m 00 mile pace for 5.00 mile pace on 3 mile run
5 m 45 mile pace for 4.45 mile pace on 3 mile run
5 m 30 mile pace for 4.30 mile pace on 3 mile run
5 m 00 mile pace for 4.15 mile pace on 3 mile run
This maffetone chart was never meant by maffetone to be used for runners it was rather a guideline where your upper end endurance pace should be to predict a 5 kilometer race time.
So if you can run at 15.40 for 5000 meters now and don't have a lot of time to run say 30 - 45 minutes usually and 1 h 15 once a week you could probably still improve if you run all those miles in 6 m 30 to 6 m 00.