mileagequestion wrote:
The only reason I did that experiment was because I read that slowing the pace is good for aerobic development.
This is not true in itself. Anything around or below the 'aerobic threshold' will specifically target aerobic development; the aerobic threshold is 'easy', but not trivial/very slow/'recovery' pace. Probably you are around that pace for your runs now. The closer to the aerobic threshold you are, the better training for the same length of run. A major goal, I would say the single most important goal, of training is to improve your aerobic threshold, and to do that you need to provide a stimulus to your aerobic system; running near the aerobic threshold is a good way to do that (it will slowly be a faster pace for the same effort and length of run over time).
A different stimulus can be obtained by slowing down a little, but running further. Time is important when it comes to aerobic development (and for tolerating the impact of running better, which is also important for long term improvement). You wouldn't want a 90 minute or 2 hour run to take too much out of you for the rest of your training, and thus you might wish to slow down a bit, but there's little benefit to slowing down beyond a point that enables you to achieve that. On the contrary it can be a very good stimulus to push yourself on such long runs too.
To get the history right, Lydiard's athletes were doing most of their training at steady/easy pace, not the trivial/'very easy'/recovery pace mentioned above.
Running above the aerobic threshold regularly also provides a big stimulus to the aerobic system, because you are stressing it significantly There are systems of training (the Verheul/easy interval method for instance) which more or less always involve running above the aerobic threshold for some of the time on a run. Again, it is a balance between intensity of stimulus and training volume.
I'd say a decent rule of thumb is to not slow down unless you have to to reach your volume goals. I.e. certainly training more will lead to improvements, and the more quality training you can do the better. But there is a limit to how much one can run at a certain level of intensity in a certain window of time; if you still wish to run more in that window, you'll need to slow down in places.
The art of training is juggling running at all kinds of paces to maximise the stimulus you are providing, of which volume at each and all of these paces is a parameter.
There are people who never run much lower than the aerobic threshold, there are people who nearly always run much lower than it, and there is everything in between! The key is to run with a purpose/stimulus in mind, not just slow down for the sake of it.