There are quite a few studies comparing constant moderate effort exercise to high intensity interval training (i.e. where you alternate short high effort intervals with moderate "cool down" intervals in the same workout. Pretty much every one I'm aware of says that interval training improves cardiovascular health and reduces various cardiovascular risk factors much more than constant effort exercise. Here's one of many: https://johnsonfrancis.org/professio...training-hiit/

From my own experience, I can tell you that doing a few intervals here or there during my riding has made me able to ride faster witless effort and has improved my endurance.