Studies show women who exercise five times a week at a moderate pace burn less fat than women who exercise three times a week at a more intense training speed. Taking classes or running on the treadmill are great, however when you start to hit that mark of "I'm bored with my workouts," or "I'm not seeing any changes," you need to switch it up.
Working at more HIIT (high intensity interval training) speed will definitely keep your body awake, keep you more motivated, and interested in your workout regimen. 
The Journal of Applied Physiology reported that young females who performed seven HIIT workouts over a period of two weeks experienced a 30% increase in both fat oxidation (fat burning) and levels of muscle enzymes that enhance fat oxidation. Just because your sweating and burning calories doesn't mean you're burning fat. Not only is there a decrease in body fat with HIIT but also in this particular case it showed that the participants' muscle fibers had significantly higher markers for fat oxidation (fat burning) than those working in a continuous steady-state exercise group.
Another study done in 2001 by East Tennessee State University found that subjects following the HIIT program burned almost 100 more calories per day during the 24 hours after exercise. Because your body is still awake from the more intense training you're performing it's still in shock and burning calories throughout your day even after your workout.
Don't over compensate trying to make it to the gym and driving yourself crazy seven times a week working out. Do a more intense training workout three to four times a week and you will have more energy and quicker results.
For example try instead of running 5 miles on the treadmill try exercise out:
25 minute HIIT on the treadmill or at the park
| Minutes | Speed |
| 0-5 | 6.0 |
| 5-6 | 8.0 |
| 6-8 | 6.0 |
| 8-9 | 8.0 |
| 9-11 | 6.0 |
| 11-12 | 8.5 |
| 13-15 | 6.0 |
| 15-16 | 8.5 |
| 16-18 | 6.0 |
| 18-19 | 8.5 |
| 19-21 | 6.0 |
| 21-25 | 3.5 |
The purpose of HIIT training is to keep your heart rate up. When your body gets used to the same workout you're not challenging yourself hard enough and that's where your body plateaus and stops burning fat.