Lydiard suggested that you could identify the best pace for your runs by doing an "out and back" test. Find a course where you can run out for half the desired length of the run and where the turnaround point is at about the same elevation as the start, i.e, where you aren't running predominantly up or down hill for half the run. Don't look at a watch in the second half until you're back. If you got back in about the same time you went out you had found a good pace. If you needed more time to get back than it took to go out you ran too fast at the start. If you get back in a lot less time than you needed going out the first half was too slow.
This was how we did it back when you didn't have any tools to facilitate pacing. It's very easy to go out too fast or too slow and deceive yourself into thinking that you got the best out of this type of progression run. The margin for error is less when you assign a goal pace. For example, a 5K runner in 17:20 shape might complete a 40 min out and back run by going out at 7:20 pace and coming back at 6:40 pace. Paces are not immovable targets and that's what you have to teach runners. Goal paces set you up for success but you still have to listen to your body and make adjustments along the way.