This is a great talk by a guy called Dr. Stephen Seiler. He has lots of resources and his own YouTube channel but this one gives you a good overview. Basically if you're looking to improve your cardio fitness, you follow what's called an 80/20 rule (otherwise known as polarised training). 80% of your training is done in Zone 2 (easy enough for you to hold a conversation) and the other 20% is high intensity. Most of the worlds best endurance athletes follow a version of this (with different specificities relevant to their sport and events).
Great little rabbit hole to go down if you're looking to improve your aerobic endurance/speed.