Another great question Alan,
I’ve been asking myself the same question for some time. I just entered the 5K Pace Plan, but I am adjusting the workouts with substitutions for heart rate and power where it “might” make sense. This because after review of my history I see that I achieve nowhere near an 80/20 ratio based on heart rate on any plan.
For example, today I ran RRe12(Recovery Run) converted to Power. I spent one minute in heart rate Zone 1, but power and pace fell into Zone 1 for power and pace both spilling into Zone 2 for a couple of minutes. I don’t think it makes any difference for a “rest” day.
I did a similar substitution earlier this week for RFF48 (Fast Finish Run). The idea was to enter the Critical Velocity fast finish segment early and as best as possible fight to reach a Zone 4 heart rate in the 5 minutes allowed for the fast finish – I failed miserably, although I was in the zone for most of power and pace.
There are a lot of variables, but your question is my question – are we leaving something on the floor when we don’t stress the heart rate on some of these workouts?