Exercise plateaus are based on the physiological principle that the body is lazy, or just misunderstood. Or better yet, efficient. Essentially, if you do not change your exercise patterns (adding new exercises, more intensity, etc.), your body will find an easier way to do them – your body will adapt. For example, if you walk or run a 3 km route 3x/week over and over again, the body will figure out a way to cheat. However, in this case, cheating is fair because this newly acquired efficiency allows the body to progress to the next level.
Overcome plateaus by introducing variety. One way to incorporate variety is to balance aerobic with anaerobic exercise. Aerobic training (easy to moderate intensity cardiovascular exercise) develops fat burning for quicker recovery from anaerobic exercise (intervals, strength training). Usually people prefer aerobic OR anaerobic training but limit their results by not doing both.
If you start an exercise session with a lot of strength and/or speed but fade quickly, you may need more aerobic exercise (ex. 20-60 minutes of running, cycling, rowing, etc.). However, if you start an exercise session slowly but improve with time, you may be anaerobically challenged. In this case, you may need to work on more interval sessions (1-3 minutes of higher intensity/speed with active recovery) and/or do more strength training.