Libby - I'm surprised that at 137 bmp you're in the red zone? I use a Polar Heart Rate Monitor which I find to be very accurate. My heart rate ranges from 150-175 bpm once I'm warmed up and into the workout. By the red zone, I'm assuming you mean you're heart rate is too high? Is this according to Mi Coach?
If you workout in a lower heart rate range, it's said that you'll burn more fat during the workout. However, if you workout at a higher heart rate range, you'll create an afterburn effect and continue to burn fat and calories for hours after a workout. You may want to reconsider your red zone. However, having said all that, if you're comfortable and will only stick to working out in a lower heart rate zone...stick to what feels good! By all means, I don't want to discourage you!