Did you have a weekend filled of long runs? Fun, fast workout at the track yesterday? Well, an easy recovery run might be better than a day off and certainly better than a moderate intensity run.
Recovery runs help the body flush toxins from the muscles which aid and speed recovery. So the days are over thinking a slow easy run is junk miles, with no purpose. Junk miles come in when you run too hard.
To achieve the purpose of these active recovery runs, you effort must remain slow and very easy. This can be the hard part for the ego, so be sure to perform these runs alone and away from an environment that might not allow you to do what you need to.
To perform these, run 30-40 min on a flat course at a very easy pace and effort. These effort is 2+ minutes per mile slower than your marathon mile pace - very easy. If you do not know your marathon pace, simply run only breathing thru your nose. Or, as an alternative, go a 30 min. barefoot run on grass nose breathing.
Note how good you feel at the end of this run. This good feeling will provide confidence for future easy, recovery run.
Your post kind of answers part of my question: why do we need the "cool down" phase? What does it do for us? If I stop moving at the end of a run my heart rate recovers quickest? But, what would I be missing if I did just stop? And, by the way, how important is Heart Rate Recovery data?