Your heart rate is a great monitor for how your cardiovascular exercise is working for you. We are talking about BPM (beats per minute). The calculation for max heart rate is 220 minus your age. So if you are 50 the calculation is 220 - 50 = 170 bpm.
The best judge of fitness is to warm up for about 3 minutes. Go into an intense version of the warm up for about 60 seconds or more. Test your heart rate for the next minute or two. If your heart rate reduces from about 140s to about the 120s within a minute you are very fit.
However, as your time doing the exercise increases the intervals get more difficult. Your heart rate may get higher and you may have trouble getting it to come back down so quickly.
This is the main focus of cardiovascular exercise. You want to take your heart rate up and bring it down. As you increase your time and intervals work your heart works harder. But that is what you want. Keep taking the heart rate up and bring it down. Practice this for a few weeks and you should get more and more fit. This type of exercise coincidentally burns more calories.