Whatever speed you end up doing, do it *early* in your training session (following a suitable warmup, of course).
Working on speed *after* a distance run is for the birds. If you want to develop the neuromuscular skill of fast efficient running, you need to treat it like any other neuromuscular skill: work on it when you're fresh, not after a lot of other activity.
However, *following* speed with moderate mileage works okay for a lot of people.