During exercise you can track how hard you are working by your heart rate. Find your maximum heart rate by using the equation 220- your age.
For moderate intensity exercise, aim for a heart rate of 64%-76% of your maximum heart rate.
For high intensity exercise, aim for a heart rate of 77%-93% of your maximum heart rate.
If you perform the same cardiovascular exercise at the same intensity and time frame, as you track your heart rate you will notice it will not get as high overtime. This is because your cardiovascular fitness is improving. You want to progress your exercise every approximate week to continue to challenge yourself and improve your cardiovascular fitness.