I mentioned this as a comment to a previous post, but I thought it deserved its own post as well. If you havent exercised in a long time and have gotten out of shape, mild interval training is a good way to rebuild endurance and cardiovascular fitness without killing yourself.

Lets say your exercise of choice is running. Maybe you can only manage to run a few minutes before your heart feels like its about to explode out of your chest and youre forced to stop for fear of passing out. Its going to take you a while to build up to 30 minutes if you simply run your maximum each day and then stop.

So instead of that, try running 1 minute then walking 1 minute, repeating this cycle for as long as you can. Youll find that you can go much further without overtaxing your heart and muscles. Maybe you can even go 10-20 minutes your first time. If run 1 walk 1 is too hard, try run 1 walk 2 or run 2 walk 3. Experiment to see what intervals work best for you. If your heart is still racing after the walking interval, increase the walking interval and/or reduce the running interval.

I recommend you aim for an initial interval that allows you to do the most amount of running in a 25 minute period. If you can only manage run 1 walk 4, thats fine. Take a week to experiment with different intervals if you need to, but find one that allows you to go 25 minutes total (even if its run 1 walk 24). You should be physically challenged but not to the point of feeling nauseous or faint.

Once youre able to go 25 minutes, gradually increase the running time and reduce the walking time. Aim to reach run 1 walk 1 for 25 minutes for an initial goal. Be patient as it may take you a few weeks to get there if youre starting with a longer walking interval.

Next, gradually increase the running period while keeping the walking period at 1 minute. Go from run 1 walk 1 to run 1:30 walk 1. Then try run 2 walk 1. Keep extending your running intervals until you can manage run 8 walk 1 for 25 minutes.

Now keep those intervals the same, and gradually build your time from 25 minutes to 45 minutes. Dont worry about distance, and dont be concerned if you run a little slower. Just put in the time. Aim to increase the total time by about 10% per week, which roughly averages out to adding about 30 seconds per day.

Once you can do run 8 walk 1 for 45 minutes, you can switch to running continuously. Youll likely find youre able to do 25 minutes continuously without any trouble. After that you can continue to increase the distance, build your speed, do harder forms of interval training, or just maintain your current routine.

The advantage of this type of interval training is that youre still spending most of the time in your aerobic range, so your circulatory system will get the benefit of that conditioning. If youre out of shape, the running intervals will spike your heart rate quickly, and your heart rate will take a while to come back down, so even while walking youll still be mostly in the aerobic range. But youll avoid burning out from having your heart rate get too high. Walking wont tire your muscles as much as running, so your legs wont give out as quickly, and you wont be quite as sore after your runs.

You can adapt this idea to other aerobic exercises as well. For swimming you can switch to a slower stroke or a glide. For bike riding you can coast instead of pedal hard.

This technique is also useful for distance running. Some people run marathons in a pattern of run 7 walk 1 (or similar intervals), and they often find their finishing times are better vs. when they run continuously. The brief walking periods slow you down in the early miles, but they make up for it in later miles by keeping your running muscles fresh. So you end up maintaining an even pace even through those last 6 miles where many people hit the wall. I once used a run 7 walk 1 pattern for a 13-mile training run, and I finished in 2 hours, which was a good time for me. I felt I was running strong all the way without dragging at the end.

Even if youre terribly out of shape, you can use mild interval training to rebuild your fitness to a healthy level without causing yourself tremendous pain and discomfort. And it probably wont take that long. A typical marathon training program can take you from running 3 miles to running 26.2 miles in six months, and that requires much more time and effort than going from 0 to 3 miles.