I've been reading that developing one's aerobic base is good for improving overall speed, and that to do so, I should be training at 60-75% of MHR. Is there still merit for overall speed in training above 75% of MHR? Or should I be saving that for trying to set a new PB once a month of something?
Background: I'm getting back in to running, I've read that one of the tests used when joining the UK military is to run 1.5 miles in <12:30, and then another 1.5 miles in <10:00. I don't particularly want to join the military, but it seemed like a good target - 3 miles in under 22:30. I'd also quite like to beat my half-marathon PB.