Regarding the original question, 9-minute pace is REALLY slow for someone who runs in the mid 16s for 5k. I would never advocate such slow running during "base training," unless you're coming back from an injury or something. It's not a pace that facilitates aerobic development. In fact, of the aerobic adaptations that we are aware of (e.g., more capillaries, mitochondria, enzymes), most of them require you to be within about 10% of marathon pace. (See, e.g., Canova and Arcelli, page 49 ("As for runs at a speed that is lower than 80% of marathon speed, they will certainly not influence performance determining factor in athletes who have already achieved a good condition.")) That doesn't mean that there aren't benefits to running slower; it just means that we haven't pinpointed them with science.
There's a place for really slow running when necessary for recovery, but typically this is later in the training cycle, when the workouts are getting a lot harder. When you're a long ways from your goal race, typically you should be doing your everyday runs at a FASTER average pace (in conjunction with much easier quality days).