1. In order to improve your fitness, you need to progressively increase the difficulty of the exercise. If you keep doing the same exact workout over and over again, your body will only improve enough to handle that workout and no further.
2. This means that your workouts should have a set of fixed parameters and a set of measurable goals. To illustrate, here are some Crossfit workouts as examples:
Angie: fixed parameters are the exercises you have to do and the number of sets/reps. The measurable goal is the total time the workout takes.
Nicole: fixed parameters are the duration of the workout and the exercises you must do. The measurable goal is how many rounds you can do in the fixed amount of time.
Lynne: fixed parameters are the exercises, the weight and the number of rounds. The measurable goals are how many reps you can do each round.
Tabata Something Else: fixed parameters are the exercises, the number of rounds, and the work/rest timing of each round. The measurable goals are the total number of reps of each exercise that you can do.
3. Each time you do a workout, you record how you did on the measurable goals. If you re-do the same workout at a later point, the idea is to improve on the measurable goals. Improved numbers in the measurable goals will reflect an improvement in your overall fitness. So if Angie took you 30 minutes the first time around but only 20 the next time, or you did 50 pull-ups in the first Tabata workout and 70 in the next one, you have a very clear picture of how your fitness has improved.
4. In fact, the improvement in your workout numbers is probably a far better reflection of your fitness than something like heart rate. That is, if your heart rate is 10bpm lower after a few weeks of exercise, it
might mean that your endurance or stamina improved, but it's hard to tell by how much. Moreover, it could also just be an indicator of lower bodyweight, that you slept more the night before, you drank less caffeine that day, etc. On the other hand, if you decrease your Angie time by 10 minutes, you know for sure that you've significantly improved your cardiovascular endurance, muscular endurance, stamina, strength, and so on.