If you are able to maintain higher intensities for a prolong series of time vs. lower intensities, performance and fitness would increase. However, there are different levels of fitness and tolerance between all of us.
Rule of thumb: Individuals who are “de-conditioned” should work out between 50-60% of their Resting Heart Rate and/or 60-70% of their maximum Heart Rate. For the “conditioned” individuals, intensities may be increased to 70-95% of the maximum Heart Rate.
In aerobic training, there is a balance to keep. Overtraining and improper technique can cause injuries. Detraining, which is reducing the training volume, can cause CRF to decrease and would require elevated intensities to regain or maintain it. Proper recovery is essential to properly allow all metabolic functions to work, allows for the body to cool down, and replenish ATP and O2 levels.