Power Walking/Treadmill - Fat Burning Heart Rate

Just need some advice/tips on this...

I recently joined the gym and have been power walking on the treadmill as my cardio (I wouldn't last more than a minute running)

Anyway, I've been doing research online and it turns out power walking gets your heart rate at the "fat burning" level. I used an online calculator and it said that my Maximum Heart Rate is 198 and my "fat burning zone" is at 138.

This seems awfully low, right ? Usually I walk on the treadmill for 60 minutes at 4.0mph at a 10.0 incline and burn around 500+ calories, but my heart rate is going into the 180s when I do that.

I just don't want to be wasting my time at the gym and want to get the most benefit out of it ! Thanks !!!