In interval training, you alternate between bursts of higher-intensity exercise and periods of less-intense exercise (or "active rest"). As you get more fit, you decrease the "rest" time and increase the high-intensity periods. You'll see big fitness gains if you train this way regularly.
For example, if you now run for 30 minutes at 6 mph, try this routine: Jog for five minutes to warm up. Then, increase your speed to 6.5 mph for one to two minutes (less if you can't go that long). Then, jog for a few minutes at your normal speed, then again at the faster speed, and so on until you reach your time limit. Your ratio of work to active rest would be 2:3 if you ran for two minutes at 6.5 mph, then jogged for three minutes at 6 mph.
You can also use your heart rate to set intervals. For example, if your heart rate hits 70% of your maximum when you jog at 6 mph, start at that speed. Then increase either your speed or elevation (if you're on a treadmill) to get your heart rate to 85% or 90% of your maximum heart rate for one to three minutes. Then, go back to jogging at the 70% heart rate, and continue alternating.
We recommend interval training just once a week to start, as it is more intense than you may be used to. Once you get a feel for it, you can do it more often.
