There are different ways to monitor your training:
One of the easiest methods is to indicate the intensity after each training session using the modified Forster scale. This scale goes from 0 to 10 and gives you the possibility to determine your subjective load using the session RPE "ratings of perceived exertion".
Average heart rate:
The heart rate is measured with a chest strap or a wrist watch. The average heart rate of a session will give you an indication of how stressful or how intense a session was for you.
This requires a watt measurement on a bicycle or on a running shoe. Watts are one of the most accurate parameters that can give you an indication of the intensity of a unit, as it measures the absolute power delivered.