Here is what I found over 34 years of training myself, and watching/helping other runners as far as the base mileage training pace is concerned:
This is actually applicable to a wide range of running ability and fitness level.
- Slower than 10:00 - not much improvement - even if that is all the runner can do for half a mile. If you are stuck there, I think the best way to get out is to run 10:00 for as long as you can, then take a walking break, until you can do it again, and do it until you have run a mile total. Then work on reducing your rest until you can run the full mile in 10:00. Then start extending the distance until you can run a few miles at 10:00 pace. This is how my wife recovers after giving birth (she is a 24:19 5 K runner).
- 9:00 - 10:00 - good pace, will help you develop some. Once you can run 6 miles at that pace, though, you get better results by increasing the pace somewhere in the direction of 8:00 before you start increasing the mileage more.
8:00 - 9:00 - solid fitness development happens here if you are comfortable in this range, increase the mileage to 8 a day.
7:00-8:00 - once you are fit enough to run this pace with a reasonable degree of comfort, that is where aerobic development miracles happen, and you can turn into a beast relative to what you used to me. OK to increase the mileage up until you just do not have the time for more. OK to keep the pace in the slower end of that range if you are doing tempos and intervals.
sub-7:00 - you will continue to develop if you can survive it, so be very careful. At this point you are starting to put some strain on your heart, muscles/tendons/joints, and the hormonal balance. You might be strong enough to take it in all dimensions, or, more common, you will have a weakness in one of those areas that will result in some form of overtraining or injury because your areas of strength make that pace feel easy. At the same time, this pace range does not offer significantly more benefits than 8:00 even if you are strong enough to handle it. Better put most of your efforts into workouts/tempos.
So in summary - I do not believe that the base mileage pace should be just a certain percentage of your race pace. It depends on what your race pace is. If it is in the slow range, your base runs should be more aggressive, and in extreme cases, you do your base runs at faster than your mile race pace in bursts with walking breaks. On the other end of the extreme, if you are world-class you can afford to be way slower than your marathon race pace in your base mileage if you do enough fast running in workouts.
I do not know why it works this way, but my guess is that base mileage training consists of multiple dimensions - muscular, cardio, nervous systems, etc - and in some of those areas there are minimum required levels of intensity you have to reach in order to improve that are not sensitive to your current fitness. E.g. perhaps running slower than 10:00 does not generate enough metabolism to stimulate adequate fat loss or muscle development, or perhaps it makes you practice inferior muscle recruitment patterns, while those barriers get overcome at sub-8:00 pace completely with no new barriers to cross as the pace gets faster.