So instead of running for 1 hour at a certain pace (best aerobic pace) he would find out how far it took to run that hour and mark it. Next time the goal is to reach that marked point faster. So this is a specific way to gradually increase intensity one step on from steady paced running. Fartlek could well be the next step.
I thought that during the base phase it was supposed to be the same effort but the times came down on their own, without any additional effort.