Depending on your fitness goal, there are some common errors that could be stalling your progress. Whether you're doing cardio to drop weight, lose body fat, train for a race or decrease stress, here are four mistakes and how to fix them fast, getting maximum results.Goal: drop weightMistake: Walking or running at one paceA leisurely stroll on the treadmill or the elliptical for a solid 45 minutes may get you sweating, which will help with temporary weight loss, but there are more effective strategies for your longer-term goals. I'm assuming that if your goal is to see the scale go down, you probably want the weight you lose to be from fat.The fix: Interval trainingInstead of shuffling along reading your magazine or watching TV, get ready to work. The bad news is you'll need to work harder than you have been. The good news is you can go for a shorter period of time. Plan on a 20- to 30-minute workout. Warm up for 5 to 10 minutes, then perform a 30s-90s interval at a pace you couldn't hold for longer than that. Your heart rate should be above 85 percent of your maximum rate, or you should be working so hard that you couldn't carry on a conversation. Follow with equal or longer recovery periods at an easier pace, letting your heart rate come down to under 70 percent of max. Repeat for five to 10 rounds.Goal: decrease body fatMistake: Doing the same thing over and over againIf you get on the same cardio machine and clock the same amount of time at the same intensity every day, you're wasting your time. Doing the same thing over and over again, your body adapts and stops changing and will not drop fat.The fix: Change it up.Change up your routine, including intervals, as mentioned above. Each time you work out, push yourself harder than the last time. If you've been using the same machine for more than four to six weeks, it's time to switch it up.Goal: train for a raceMistake: Using the treadmillThis doesn't work as well as hitting the streets, unless your race is on a treadmill (doubtful). The problem with using the treadmill is that the ground is moving for you so you do not have to use the muscles on the back of your legs to pull yourself forward -- you just pick up your legs, which is very different biomechanically than running outside. Also, you miss out on running downhill, which makes it very hard to keep your running technique and takes practice. Without getting off the treadmill, you won't learn how to handle the force of going downhill. This could set you up to injure yourself in your race.The fix: Train on the same terrain as your race.Hit the streets or the trails to get ready. You should also look up the elevation of the race and train for hills, uphill and downhill, if necessary.Goal: stress relief and/or active recoveryMistake: Pushing yourself too hardDoing steady-state cardio and getting your blood flowing can be beneficial, but keep your intensity to under 70 percent of max heart rate.The fix: Go outside.Instead of being at the gym indoors, go outside and enjoy your surroundings with a leisurely walk. Sunshine has been shown to help decrease cortisol, the stress hormone.