What type of cardiovascular exercise are you doing? And more importantly are you doing it correctly?
You must do intervals to get the adequate work for your heart and cardiovascular system.
A proper interval will be working hard as you can for 1 minute or less and rest work for 1 minute 30 seconds or longer. One minute for the hard work is a magic number. If you are working longer than 1 minute you are most likely not working hard enough. When you work hard enough you are breathing hard and tired at the end of the interval. You recoup and do another interval if you are able in 1 minute 30 seconds or 2 minutes.
By doing this you get your heart working hard enough to pump blood and get oxygen moving through the system better.
Coincidentally you are going to burn more calories which is great for weight loss if that is your goal.
On a side note do not train for longer than 20-30 minutes. Any more time than that and you actually defeat the purpose. This does not apply to marathon runners. However, marathon runners can benefit from intervals as well because they help build endurance.
Try it. Let me know how this works for you!
Happy training, Kathie