I just started doing cardio this way. To get that optimum fat burn for 45 minutes to an hour, my heart rate needs to be between 131 and 141. The problem is at my gym the little handles on the treadmills that are supposed to measure your heart rate when you grip them don't work... so what's a good pace? I'm currently doing 3.7-4 on a 1.5 incline. Is this sufficient?