I'm an ultra runner, so I'm probably not a good one to answer compared to most people here. :-) But I think a different perspective can be helpful...
First, I'm not chronic cardio. I run marathons on as little as 15 miles per week average, and utlra's up to 40-42 miles on less than 30. I'm shooting for 50 in the fall and probably won't average more than 30 miles per week leading up to that.
I mix in other work like crossfit, movnat, lots of hiking, a little mtn biking, some paddling, traditional weights, etc., to make it all happen.
I consider running part of my "play time" as outlined by Sisson. Is that an excuse, or is that cheating? I don't know, but I love to run, so it works for me...
I do think that 6 days of cardio with HRM at 80%++ of Max is not so good for you. On the other hand, I think training at your maximum aerobic rate, is not as bad for you as the chronic cardio camp says it is. Much of my running is at or just below my max aerobic. But I will mix in anaerobic work in the form of races and intervals, but only for a small period of time.
For many people, runners included, max aerobic rate is going to feel awfully slow when 1st attempted. My max is around 135-145 bpm, which is a 9:30 - 10:30 pace depending on how far into the run I am. However, my 5k race pace is just above 6:30 and marathon pace just under 8:00.
I run mostly by feel, but will use an HRM every now and then to ensure my "feel" has not drifted from reality.