My friend and I were just talking about this today and we both concluded that although we're both healthy and active (I run/spin/strength train daily and do 10-12 miles runs or half-Iron bricks on the weekends), we both burn the most percentage of fat calories ... walking! I'm a huge fan. You mention the study says "low intensity" walks may not work and I think that's the key -- I think a higher-intensity walk of sufficient duration that elevates the heart rate into the aerobic zone does provide health benefits. ("Sufficient duration," from what I read, is considered to be at least 30 minutes of high-intensity exercise, like running, or 50 minutes of lower-intensity exercise, like walking, at least three times a week.) Like you say, I think it is necessary to focus on the effort -- whatever "effort" might be to each walker -- in order to truly benefit from the activity. Anecdotally, I'd say walking works for those who do put in the effort -- I know two people who have both lost 50+ pounds simply by walking. (Neither likes running.)