Intervals provide the best results and provide an opportunity for the body to recover from short high intensity periods. This is why I love hilly countryside as intervals occur naturally. There is an old adage that if you can talk in a normal manner then you are not pushing hard enough and in general that is true.

A stress test can show a heart arrhythmia although most people have an erratic pulse rate and it is over diagnosed and a lot of unnecessary surgeries often result. A blood pressure test that compares pressures at the top and the bottom of the body done by a competent individual is the best tool to see if there is a problem.

I also adjust my heart rate based on the temperature and the humidity level. I worry more about heatstroke than a heart attack.