the fat burn will keep you right around 65% of your max heart rate, and is usually done at a slow jog, or a fast walk.
i suggest intervals. you will burn more cals faster, plus it will keep HGH levels up… and you’re getting old now, so they are starting to drop.
oh, and buy my tread
oh yeah, and to mimic running out doors, set the incline to 1-2 deg, or stop being a pussy and use the hill program that every tread in the world has.