It is possible that, given your age and regular work-outs, your cardiac conditioning is fairly good and you simply need more exertion to raise your heart rate adequately(the more fit you are, often the slower your heart rate). 20 minutes in may not be a long enough time. The other likely factor may be medications you are taking which may prevent your heart rate from going up(examples are metoprolol or propanolol). If you have other symptoms like lightheadedness, easy fatigue or dizzy spells, you should speak with your doctor of a cardiologist to ensure the electrical circuits in your heart are doing what they should.
Hope this helps,