If I want to increase my fitness level, how long should I do cardio for?

Consistency is key. Improving your fitness level happens over a period of time and not in one workout. You will want to listen to your body and feel free to adjust your time depending on how you feel that day. Interval training is extremely effective and there are a few other strategies you will want to keep in mind on your journey. The F.I.T.T.E. (fight) principle is a good guideline to follow. F- stands for frequency. Usually broken up over a week or a month. You want your frequency to be daily or every other day at least. This is the consistency you want to work on for improving your level of fitness. I- stands for intensity. This is the pace you are going at and how hard you are actually training. Some people like to go as hard as they can every day but this is not important.

Again, listening to your body and going in intervals is a good method. Some days your intensity can be your maximum and some days can be more moderate. You can also break up your intensity during your one session by minutes. The first T stands for Time- your workouts can last for 15 minutes or 90 minutes. Some work out for longer and this is a personal preference. To increase your level of fitness you want to try for 6-9 hours a week. Paying attention to how you are feeling before, during and after each workout. T- the second T stands for TYPE. What type of exercise will you be doing? you can walk, run, lift weights, swim laps, ride a bike; the types of exercises you can do are endless and you should mix them up to keep your body guessing. It will respond by getting stronger if you are able to change your work out on a regular basis. The E- stands for enjoyment. If you are trying to do something you do not enjoy you are raising your risk of dropping out of your fitness program completely. It's ok to try MANY different types of fitness until you find one that you actually enjoy. 

In order to increase your fitness level in terms of cardio, you can progress through the incorporation of intervals into your training. The average healthy adult should be performing cardio at an intensity level of 60 - 75%, however as your body adapts to this intensity level you should progress to the second stage of cardio training with the incorporation of interval training. Upon advancement to the second stage of training, you should begin incorporating short intervals at a higher intensity level of 80 - 85% of your maximum heart rate after which you recover at 60 - 75%. As you improve and your body adapts, you should increase the duration of your higher intensity levels and decrease your time spent in the recovery zone. Eventually, your body will adapt to this type of cardio training as well, however you can continue to improve your fitness levels by advancing to the third stage of cardio training. In this advanced stage, you will begin incorporating intensity levels of 86 - 90%. In this type of training, you could spend one minute in 86 - 90% zone and then reduce your workload to 80 - 85% for one minute and then completely recover for 10 minutes in the heart rate zone of 60 - 75%.

Robert S. Kaufmann, MD
Internal Medicine
To increase your fitness level you must keep increasing the time and resistance. Everybody starts at a baseline and with consistant exercise that will improve.  An example a person runs a mile and it takes 20 minutes as they keep running that time starts decreaseing and before you know it you are running 1.5 miles in the same 20 minutes.  The main thing is to keep exercising and keep pushing yourself a little extra each time.
To improve your fitness levels it's recommended you perform 20-60 minutes of cardio exercise at an intensity 60-90% maximal heart rate (220-age = maximal heart rate), 3-5 days a week. Work on building a strong and solid cardio foundaton first by performing 30-60 mins of cardio exercise at 60-75% maximal heart rate. When you can do this and maintain a steady heart rate regularly, then work toward increasing the intensity of your workouts by adding interval training into your cardio workout plan.

I recommended three or four times a week for 30 or 40 minutes depending on the intensity of the work out and your current fitness level.  You will be able to gauge your improvements by monitoring your time, speed, distance, and intensity levels.

It is recommended that you perform 30 minutes of cardio at 65-75% of your maximum heart rate (220-age=maximum heart rate) a minimum of 3 times per week.  Thirty minutes maybe too hard for those that are just starting out with a cardio program,.  In that case break that 30 minutes into three 10 minute sessions for each day you do cardio.  Once you are able to perform a full 30 minutes with a steady heart rate, it’s time to add interval training to your program.  Start with 5 minutes at 65-75% max heart rate (this is classified as zone 1).  Increase the intensity of the cardio workout until you reach 80-85% max heart rate (zone 2) and hold for 1 minute.  Return to zone 1 for 5 minutes and repeat the cycle.  As you progress, keep your heart rate in zone 2 longer and eventually add zone 3 (86-90% max heart rate) to your interval training. 

The key to improving your fitness level, or cardiovascular fitness, is quality, not quantity (time).  I recommend at least 30 minutes per day, 3-5 days per week.  Challenge yourself and keep changing up your cardio exercise, by increasing the difficulty level, changing the type of cardio, and incorporating interval training, which was described in some of the other experts’ answers.  What you don’t want to do is the exact same thing day in and day out!

One important note is not to over train yourself with high heart rate cardio.  If you do a high-intensity interval workout where you are hitting near your maximum heart rate (in other words, you are doing intervals where for the hard intervals you are pushing yourself as hard as you can – also known as level 3 cardio), do a lower-intensity cardio workout the next day.  Do not do level 3 cardio multiple days in a row.   Like other muscles, your heart muscle needs some recovery time when pushed to the max!

Good luck, and enjoy making your heart stronger and healthier.
Your cardio sessions should last at least 10 minutes at a time. But the longer you are able to exercise, the more calories you will burn and the more endurance you will build. Aim to slowly increase the duration of your physical activity over time to reach at least 30 minutes of continuous cardio. If you can eventually go even longer - for example, 60 minutes – that is even better.
According to the Centers for Disease Control and Prevention, your long-term goal is to do cardio exercise at least five times per week. Discuss your exercise plan with your physician if you are just starting out or are significantly changing your activities. Your physician can give you exercise recommendations based on your current fitness level, health history, and physical fitness goals.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.