I touched on this before but to briefly summarise- yes there is data to suggest that exercising at a lower intensity will elicit a higher degree of fat utilisation, no one is going to argue with that, but when you look at the bigger picture i.e. Total energy expenditure, my personal opinion is that your best option is to go for the biggest calorie burner you can, relative to the time you have and attempt to make the biggest dent in your daily energy expenditure possible.
Anecdotally speaking, with diet being the biggest determining factor in weight loss, if you only have fifty minutes to exercise, do you really want to be spending the majority of that maintaining a steady heart rate on the X-stair-master-blaster-5000 in order to stay in ‘the fat burning zone’, or do you want to be focussing on big, bang for your buck resistance movements that are going to have a profound effect on your body composition?
Remember, intensity is the inverse of duration when we’re talking about calorie burn, so ramping it right up to ludicrous speed for the last 10-15mins of your workout with a spicy finisher really can have a dramatic effect on your total caloric expenditure.
Increasing your NEPA (non exercise physical activity) is a great way to get in some steady state work across the board, but if you’re interested in being time savvy, stop trying to lovingly woo the fat away with long, romantic walks and start evicting it from your body like the useless, rent dodging tenant it is, with zero impunity.