I'm wondering this myself. Does anyone know? I doubt it. If you ask any of the top exercise physiologists of the world, they will tell you that there is no conclusive evidence that there is a minimum pace that elicits improvement of the aerobic conditioning.
Some claim and I also believe that it does not matter how slow you run your easy runs, what matters is how long those runs are as well as how high the weekly mileage is.
Here is my take on it. What we are trying to do when we run slowly is increase the oxidative capacities of our slow twitch muscle fibers. We do not need to run very fast to recruit these muscle fibers, so we don't need to be picky about the pace, we just need to run easy. The only thing we have to worry about is the distance.
We can't just run slow all the time though. If we ran slow all the time we would not be maximizing our potential. We also need to do some faster running at speeds that will recruit our type IIA fast twitch muscle fibers. By training in this way, we can also increase the oxidative capacities of these muscle fibers. The minimum pace required to recruit these muscle fibers seems to be about 15-30 seconds slower than 10k race pace. We do not need to do a lot of training at these faster speeds to see improvement, but we should do some, probably some where around 10 - 15% of our weekly mileage. Doing more than this on a consistent basis tends to cause us to break down in stead of build up. The speed and duration that we perform these faster runs will depend on the event you are training for, since we also need to train our muscles to become more efficient at the speed we are going to race. A lot of coaches recommend training no faster than and no slower than 10% of goal race pace. Doing your race specific training any slower or faster than this does not seem to improve performance in your event of choice.
Does this make sense? I believe training really is as simple as:
1.Run moderately high mileage
2.Take it easy on your long runs and recovery runs
3.Train at race pace
4.Work on your sprint speed
Not much else to it. I believe staying motivated and injury free is the hard part.