High Intensity Interval Training (HIIT for short) has been shown in countless studies to increase your metabolism for up to 36 hours. The problem with HIIT workouts is, they are not a heck of a lot of fun, and many people find themselves having a hard time getting motivated for these types of high intensity routines after starting the program. Again, these high intensity training workouts are proven to be extremely effective but motivation is often a big drawback. Have you started on your first HIIT yet?
The weight lifting aspect of many HIIT's increases the size of your muscles and larger muscles burn more calories (increasing your metabolism) all day long. It's all but impossible for women to get large muscles (think body builder) without some form of chemical assistance. I'm guessing that if you are here to read about HIIT's that you are most likely against trying to 'cheat' your body, right?
A good HIIT training regimen will of course include an equally good diet, as your body will need proper nutrition and energy reserves to complete these HIIT workouts. Low Carb / High Fat diets have been shown in numerous studies to increase your metabolism by 10-15% (at least for most people). Low carb diets in this case indicate to around 10-15% of your daily calories, high fat would be 70-80% of your daily calories. The rest of your daily diet would have made up of (lean) proteins. Are you ready to take your workout to the next level?
As a side note, attempting to boost your metabolism with medications, pills and chemically (legally and illegally) is most likely a very poor decision. Most of these artificial metabolism boosters have been shown to have very dangerous side effects.
People as a whole spend a lot of time in the gym doing things incorrectly, and it shows.
I see guys who spend up to 12 hours a week in the gym, but have not changed their appearance or physical ability much or at all. Cardio for 90 minutes is overkill. The best way to do cardio is in short intervals. 30 seconds of high intensity followed by 60-90 seconds of recuperation. Repeat the process 10-20 times for a total time of 15-40 minutes of 'high intensity' cardio.
As your condition increases, you can adjust the interviews as necessary. But most importantly, your diet will be the key to losing weight. You've lost the first 50 or so pounds, but the next 50 or so you want to lose will be more difficult, and as you continue losing weight, will only become even more difficult. If you have let yourself balloon up to 240 or more, I'm assuming you have some problems with self-control. Do not you think that if you learn to control your diet it will make the process of shaping your body much easier?
The BEST way to plan a new training regimen, especially a HIIT routine which can be very physically demanding on the body, is to plan a good deal of time in advance to meet your goals. It is not enough to say 'By next week I want to look like X "or" By next month I want to drop 30 pounds "but incorrect as that is, that is how many people plan their fitness regimens. to plan out in advance a minimum of 90 days. It seems that 90 days is the typical length of time needed to make a significant impact on your physique without the use of drugs or reckless techniques like sub-800 calorie diets.
One of the best places to start (pre HIIT routine) is to clean up your diet. The easiest thing to do here is to watch your portions and start eating only single servings of whatever food you are eating. Increasing your physical activity through running / jogging / walking / weight training will help motivate you to keep going. With this type of pre-planning, you are sure to get your next (or first) HIIT routine off to a flying start as well as easier to stick with all the way through to the end.