When planning an aerobic training session, the most important metric you should consider is your heart rate. Sure, time and intensity markers like minutes/ km and watts are important, but how hard you're stressing your heart should be the first thing you determine and should correlate to the workout's objective.
The reason you'll want to plan your heart rate range is due to the close relationship between the heart rate and oxygen consumption, especially when the heart rate intensity is above 50% of your functional capacity, which is your VO2Max or also know as your heart rate reserve (HRR). The most accurate way of programming intensity
The most accurate way would be to get your VO2MAX tested in a laboratory and determine how it relates to your lactate threshold, which will give you an accurate layout for zone training intensities - something the other calculations don't take into consideration, making them much less accurate. This type of testing is quite expensive. Unless you're training for a specific event like a marathon or triathlon where you'll want to have these zones dialled in as accurately as possible, I don't think you really need it.
Another way is to utilize your age-predicated max heart rate: 220 subtracted by your age and multiplied by the zone intensity. I'm not a fan of using this method, because I don't feel as though its personalized enough. I'm the same age as two of the greatest athletes of all time, Lebron James and Eliud Kipchoge - do you really think we should be using the same numbers for our conditioning? I don't.
Heart rate reserve (HRR) is a much better metric to utilize, because it takes into consideration your resting heart rate. To calculate your HRR you'll take the max heart rate and subtract your resting heart rate. Then, use the Karvonen formula to calculate your training zones. Here is the Karvonen Formula:
(220 - age - resting heart rate) x intensity + resting heart rate = Target heart rate
When you're planning your session, you should plan for a range of heart rates within one specific zone. Here is a simple chart outlining the different training zones:
% of Max Heart Rate
50 - 60%
60 - 70%
Improves Aerobic capacity
70 - 80%
lactic acid threshold
80 - 90%
Maximum capacity, VO2MAX
90 - 100%
Now lets see a couple examples of using each method and how different the intensity numbers can be. I'll use myself as an example: I'm a 38 year old with a resting heart rate of 55bpm and I want to improve my aerobic capacity, so I should train at 70 - 80% of my max heart rate. Here is how I would calculate the intensity by using just the age predicted max heart rate (220-38) and multiply it by 70% and 80%:
Low end of zone 1:
182 x 0.70
High end of zone 1:
182 x 0.80
Now, lets see what my numbers would be if I were factor in my heart rate reserve in the Karvonen formula:
Low end of Zone 1
(220 - 38 - 55) x 0.50 + 55
(127) x 0.70 + 55
High end of Zone 1
(220 - 38 - 55) x 0.60 + 55
(127) x 0.80 + 55
So, as you can see - that is a huge difference in heart rate training zones. By not factoring my resting heart rate, I probably wouldn't be improving my aerobic capacity, because the intensity isn't high enough to train that attribute. In fact, the first example may be within an entirely different training zone altogether. Your best bet is to use your HRR and plug it into the Karvonen Formula to plan your cardio sessions.
Let me know if have any questions or think I'm full of shit in the comment section.
(2) Baechle, Thomas. R, Earle, Roger W., 2008 Essentials of Strength Training and Conditioning, Third Edition, Human Kinetics