The best type of exercise for endurance and cardiovascular health goes by many different names. I like High Intensity, Short Duration or HISD.
Very simply this means you may choose any type of exercise you like such as walking, running, biking, swimming or whatever.
Let's use swimming as an example. You would swim at a rate of 90+% of your fastest swim for 30 seconds then swim very slowly or not at all for 90 seconds then repeat. Your goal is 8 repetitions. Good luck. This is not easy.
Once you build up to comfortably doing 8 repetitions of your chosen activity, you can switch to a different activity or continue the one you're doing with increased difficulty.
For example, if you chose running and you are able to do 8 repetitions of 30 seconds hard with 90 seconds rest, begin to reduce your rest time with a goal of 60 seconds rest or even less. Or, increase your difficulty by running hills.
There is really no end to the progressions you may use with this technique. Enjoy.