Walking for exercise carries a long history, as far back as 400 bc:
 
— “Walking is man’s best medicine.”  Hippocrates (460bc—370bc) 

A Harvard Medical School study concluded that a daily 15 minute walk can extend ones life by 3 years. (“15 minutes a day ups your lifespan by 3 years,” Harvard Health Letter, December, 2013. health.harvard.edu.)  Not to be undone, a British study upped the ante, concluding that a brisk 25 minute walk daily “can add seven years to your life.“  (“A Daily Walk ‘can add seven years to your life,” by Paul Peachly, Independent, August 31, 2015. independent.co.uk})  If that be true, why don’t more people walk longer — and brisker?  Obviously, A quick drive works better than a brisk walk for most busy or lazy folks.  

What constitutes a brisk walk?  The collective thinking confirms a pace that moves your heart rate into its target heart rate zone, but the quantifying measure for that zone varies.  Some examples:  (a) a pace that allows talking normally but not singing.  (CDC.org)  (b) 2.7 miles per hour (mph), a 22 minute mile. (“How fast is a Health ‘Brisk walk?” (Today’s Daily Briefing July 8, 2018)  (c) 3.5 mph, or 100 steps per minute, a 17.5 minute mile  (“What Does Brisk Walking Mean?” Healthcorps, December 6, 2013 healthcorps.com) or (d) 220 beats/minute – age X .70% maximum heart rate —a more complicated formula based on a body’s resting and maximum heart rate, (“Exercise Intensity:  How to Measure It,” by Mayo Clinic staff.  mayoclinic.org) The latter measure (d) should be more accurate, because it includes the physical ability and age of the walker.

Harry Truman (1884—1972,) former president of the United States, took a brisk walk every day, and lived to age 88.  “He rose early, took a shot of whiskey, with a glass of OJ as a chaser, and walked ten blocks at a fast clip of 120 paces per minute.   (trumanlittlewhitehouse.com)  Interestingly, none of the studies recommend whiskey.  

Not only is walking good exercise, but also can relieve crippling anxiety and stress:
 
— “The best remedy for a short temper is a long walk.”  Jacqueline Schiff, American clinical Psychologist

Writers, composers, artists think better during a walk:
 
— “Solvitur ambulando” (It is solved by walking.)  St Augustine (354—430.)
 
— “Only those thoughts that come from walking have any value.”  Friedrich Nietzsche (1844—1900.)
 
—“I have walked myself into my best thoughts…”  Soren Kierkegaard (1813— 1855.)

Moral: A walk a day yields longer life, less stress and better thoughts.