[bluecalm]

Walking 5km/h at 70kg is 350kcal/hour. If you are heavy and unfit then it will maybe be 3km/h at 90kg 270kcal/hour but then someone that unfit is not going to be able to burn 500kcal in an hour in a gym anyway.

>>IOW, the total calorie burn in a day that includes 4 hours of walking is likely relatively comparable to the total calorie burn in a day that includes an hour of gym time.

It's a good point about the delta (the estimations are for a total burnt during activity not extra) but I don't think you can get the numbers close. People greatly overestimate how much you burn during gym session as well unless they are already very fit and move constantly during that session.