Thanks for the thorough response. I've seen the same debates on Runner's World forums more than ten years ago, ha!
Again, I need to remind you that we aren't talking about a marathon, or even a 5k. We're talking about a 9 min run, by a guy who one assumes already gets plenty of longer (time) low heart rate activity on a bicycle (since this discussion is taking place here). He also isn't trying to do anything terribly crazy as far as pace goes. And over a 9 min race, he's using totally different body processes than one uses in longer races. I just think that if you want to run 1.5 miles at 6 min mile pace, you'd better get your body used to running at that pace or faster. You don't do anything of the sort by jogging around at conversational pace.
But we've got a world where people decide to start with a 6 hour marathon and then run 100+ mile weeks and hope to knock it down to 5 hrs. and then 4:30 and so on. I've always felt it would make a lot more sense to shoot for "good" times at shorter races and then work up to longer ones. If I was trying to reach the same goal as the OP, I'd go see if I could run a 400 in 1:30. Then an 800 in 3 min. Then I'd work on getting to 1 mile in 6 min. Then I'd shoot for the 1.5 in 9 min.
To do this, he may indeed get benefit from a few 5 mile runs a week - but I'd do them at a faster pace than is currently in vogue for those trying to run a long way (marathon) no matter how slow.