although people talk about doing cardio in the 'fat burning zone', it takes longer to burn the same amount of calories that you would burn if you did interval training, tempo work, wind sprints, or what have you as part of a high-intensity workout. so, while you may burn a higher proportion of 'fat calories' doing 6 mph on the treadmill, you'd have to go for twice as long to burn them as if you did 8.5 mph sprints interspaced with 5.5 mph jogs. at least that's my understanding of it. thus, unless you really enjoy cardio or are afraid of feeling the burn in your legs and lungs, i would think that more intense workouts are a better way to go. it's probably a good idea to have a couple of longer, easier cardio sessions as part of your workout, too, so you don't overtrain and for cardiovascular endurance.