Just a mention of my sub-8 minute mile from last May happened with almost zero running prior to it, and only two track sessions of repeat 400's. My walks were of about 40 miles weekly through the spring. The year before, I ran a 7:38 mile as part of KC Corporate Challenge and so several men in my age group for serious competition. My time trial mile had my wonderfully paced wife pacing me through it alone.
At my best, my walks were just under 15:00, but not always, and my limited talent had me run a 2:54 marathon in my prime (age 33) so my aerobic and biomechanical talents are limited.
I will turn 70 (yikes!) in May, and thinking of just one more sub-8. Have not been to the track of late, but hope to restart soon. Last week was a good week on my feet, 43 miles and of that, 25 were running short intervals of anywhere from 1/6 to 1.0 miles. The "runs" are, at best, 9:30, most slower.
I rarely look at my Garmin for heartrates since it's usually off from what a chest strap HR would show, but it seems that aggressive walking gets my HR up about 30 beats/min over resting, a low-level aerobic benefit (but is quite adequate for a cardiac patient, OTOH).
Walking is slower, but does allow me to look and listen to all around me more, and is relaxing, just takes longer to burn those calories and keep the pounds off. Feels hard to run longer now since the relaxed pace of walking is nice, and doesn't strain the hammies! :)