If you are trying to burn off those extra pounds without building lean muscle mass underneath that fat you're burning off, the likelihood that you will gain back all the weight you lost will be quite high. Including strength training in your workout routine builds lean body mass, which in turn will raise your resting metabolic rate, or rate at which you burn calories doing normal activity. Stronger muscles mean more calories burned throughout the day, because muscles need energy to move. If all you do in the gym is run, then you are only burning calories and only temporarily raising your resting metabolic rate for about 24 hours. In general only 15 extra calories are burned resting for every 100 calories you burn on the treadmill.