Cardiovascular Endurance is the ability of the heart, blood vessels, and lungs to supply oxygen to working muscles.
To improve your cardio fitness you need to move, and move a lot. The more you move, and the faster you move, the greater the improvement will be on your cardiovascular system. Moving can include jogging, walking, swimming, cycling, rowing, climbing stairs, cardio machines such as ellipticals, hiking, etc. The only limit is your imagination.
To see improvements you need to do this at the absolute minimum 4 times per week. Of course if you're just starting out you will notice improvements even if you only do it 2-3 times per week, but this needs to increase as you become more comfortable.
At first don't worry too much about the intensity. However, as you become more comfortable you need to aim at getting your heart rate to atleast 65% of its maximum heart rate. As a rough estimate your maximum heart rate is 220 - age. so, 65% of that is (220 - age) * 65%. Example, if you are 40 years of age, (220 - 40) * 65%. This equals to 117 beats per minute. The best way to measure your heart rate is by taking your pulse. You can find this on your wrist on the side of the thumb in the little grove. Or on your neck on either side of the wind pipe. Count the number of beats for 30 seconds then multiply by 2. If it's too low walk a bit faster.
Now, if this is too complicated for you then you can observe your breathing rate. If you can carry on a conversation without any difficulty then you're not working hard enough. if you can barely carry on a conversation then you're probably working at the proper intensity.
Another option for measuring your heart rate is to purchase a heart rate monitor. This is a strap that goes around your chest, which senses the electrical signals from your heart and transmits that to the display unit, usually a wrist watch.
If you are a real go-getter you can work harder than 65%, but working much harder won't provide any extra benefit. If your heart rate is too high then you are not allowing enough time between heart contractions for blood to fully fill the heart chambers. So in fact circulation will be impaired if heart rate is too high. This is not going to cause you any harm, but it simply won't be as beneficial as you might think.
Some experts state that working at a lower rate, e.g. 50%-60% of your max is better at burning more fat compared to other energy sources. However, what they don't tell you is that you need to workout longer, and that at higher intensity (faster walking) your raise your metabolism more which leads to more fat burning even after your activity.
Walking is probably the simplest form of exercise anyone can do. To start with, of all the exercises, walking is the most natural. You've been doing it since you were just a few months old. It can be done just about anywhere.
Now, go find yourself a good pair of walking shoes. Do not walk in dress shoes, or really thin shoes. Do not walk in boots. You need something that provides proper support, is flexible, and has soft soles so you feel comfortable. If you are not comfortable you're not going to keep it up.
Decide on your route. Ideally walking in a park is best but that is not always possible. If you will be walking in the city try and find a route that doesn't have too much traffic. Some people like to have several different routes just to avoid boredom.
Find some friends to walk with. This is not necessary, but if you suffer from poor motivation making it a social activity will increase your chance of keeping it up.
If you feel you don't have enough time to do the full walking workout at once then split it up. E.g. walk half of the time in the morning and the other half in the evening.
Make sure you have appropriate clothing. You can still walk in the winter, just make sure you dress appropriately so you enjoy the walking.
Depending on your fitness level you can start by walking just 10 minutes a day, then work up from there. If you are already fit start with 20 or 30 minutes and work up to a level you are comfortable with. If you're doing 30 to 40 minutes then you're doing great. If you're going to walk longer than 30 minutes, or it's very warm outside, then take some water with you.
Other ways you can increase the resistance is by walking faster or walking up hill. If you want to make it even harder carry 2-3lbs hand weights. Maybe combine walking with jogging, e.g. 5 minutes walking with 2 minutes of jogging. Use your imagination.
Caution: listening to music with earphones makes the time go by faster, but be careful of traffic and other dangers. You now have one less sense to warn you of danger.
Other ideas for cardiovascular exercise are running, cycling, swimming, squash, tennis, etc... anything that gets your body moving In the gym you can also get into cardio classes, spin classes, rowing, stationary bikes, ellipticals, and stair climbing.