Your predicted heart rate
on a treadmill with good intensity exercise
should be 70% of (220 - your age).
If you are 20, then your heart rate should be 70% of 200, or 140. If your heart is going 150, then you're a little bit above your target heart rate, but below your maximal heart rate of 200. As you get better conditioned, your heart rate for a given level of exertion should go down.