Want to increase the amount of fat you're burning? Try interval training. A 30-minute high-intensity interval session can burn just as much fast as a 60-minute easy aerobic run!
Distance running is great, but if you're not careful you can risk losing some of your lean muscle mass. This is called muscle catabolism. It happens when you deplete your carb stores. If you haven't trained your body to kick-in using fat as a fuel source, it will move on to the next best thing—your muscle tissue. Low-volume sprint training (intervals) when added to your regular weekly running routine can actually cause the opposite effect—building muscle (anabolism). That's because interval training helps the body produce human growth hormone naturally which helps in the muscle-tissue-building process. Adding some light resistance training can also help override that catabolic effect.
Some of you might be saying, "Well, I'd rather run longer and easier than shorter and harder if they both burn the same amount of calories. But keep in mind that after that 60-minute long-run, those calories cease to burn. After the 30-minute interval session, however, you continue to burn calories for about 24hrs! So, that weekly interval workout can do wonders to boost your metabolism.
Don't have a track nearby? No problem. Intervals don't have to be limited to the track. The simplest form of an interval can be a fartlek. A fartlek is just a burst of speed thrown into a regular run. Fartleks can be for a period of time or for a certain distance. For example, you could decide to run fast for 1 minute or you could decide to run fast to the next intersection several times during a given run. Throw 4 or 5 fartleks into a regular 5-miler and you've got a good interval workout. Hill workouts are another great way to add some intensity to your workouts. You can also do intervals on a treadmill, elliptical machine, row machine, or stationary bike!
So, add some variety to your weekly workouts, up your caloric burn, and preserve your lean muscle mass by adding some speed work into the mix.