consortium11 have already made a good post, but here is some more :)
Intensity of training;
The intensity can differ quite a lot between different clubs and individual ambitions. There are people showing up at a gym about once a week, talk more than train while there, and think they are training a martial art. Some clubs allows it, some don't. Competition focused clubs tend to not allow it, either you are focused on the training, or you step out.
Even if it is a competition focused club, it can still be a friendly place and people who are just into it for exercise are more than welcome (and often are the bulk of the members). There are those that shows up once of twice a week, and those who trains several times a week. Some focus on their technique, some pushes themselves.
Mind over matter -- ie the pain threshold, if any
Things like the pain threshold increases, often without one noticing. It might not be until training with newcomers one realize the difference. But then there is also things like learning just to keep ones eyes open. To be able to keep looking at the opponent, while punches are coming once way tend to require some training.
Extremism -- how often, if at all, do people push themselves to the extreme
Rarely to the extreme. Training to hard breaks down more than it builds up (one of the reasons so many catches a cold a few weeks after they start training something, as they start out to hard). But there is a point to push hard at times in martial arts, as another Mind over matter point, we might have to learn not to give up too soon and pushing oneself is a way to realize one can push oneself quite a bit further.
What does the training entail?
The meat of the training tend to be a lot of pad works. Throwing punches and kicks against another student holding pads, usually in a combination given by the teacher. It might also mean hitting a bag, but to not let it bounce around to much it might still be a two-and-two exercise while another student hold the bag. Those just into it for the exercise tend to only do this part.
Then there is sparring. Sparring is still training and tend to be held at perhaps 50% of ones capacity (perhaps because, how do you actually measure?).
Is the diet different?
To get the most out of ones exercise, one need to eat varied and healthy food. But there is no specific diet for martial arts.
How would one elevate themselves from merely 'training' to participating in fights (as seen on TV -- I've never watched one)
Deciding one wants to do it, then train with that focus (knowing one soon will enter a ring with someone intended to beat you apparently can be quite a motivator). To fight at a professional level, one needs to practice a lot.
How would a woman's training differ from that of a man, if at all?
At the two clubs I'm familiar with (both very competition focused), there is close to the same amount of female students as male students. The only difference is that women and men uses separate changing rooms.
How many styles are there? I have been informed that there are MANY styles!
Here in Sweden, we usually just tend to call the Dutch style for kickboxing; but we call it "low kick" when specifying it (and no, that is not a translation, it is English we are using for that part). Most such clubs are doing both kickboxing and Thai boxing (which they see as kickboxing with knees and perhaps elbows, depending on local rules). A Muay Thai club tend to add a bit of the traditional bits as well. There seems to be a bit of difference in how kickboxing clubs and a Muay Thai clubs teaches the boxing parts.
Full contact karate styles, Savate (French kickboxing), Chinese full contact styles, etc is usually refered to their own names and not as kickboxing here, at least not to my knowledge.
Then there it kickboxing inspired boxercise... Those I have seen not only not removes bad habbits, but actually adds a few more. I hope I have just been unlucky, and they're not representative.
How would you choose one from another? Are they all similar? If not, how different are they?
Availability, attitude at the club, what one enjoys. If one is lucky enough to live in a city with a lot of options, it is clearly worth it to shop around. It depends on what one counts as part under the kickboxing umbrella. So if it is as limited as here, then most clubs are fairly similar style wise. If one means "full contact standing style with kicks", then it can be very different.
As a beginning, where and how would one start?
Generally, just find a club and show up at and ask. At least here, people are welcome to sit down and watch a lesson, and usually can practice for free once before start paying. But as mentioned above, the attitude at the club can differ quite a bit. From chest thumping testosterone poisoned, to friendly and relaxed or friendly but dedicated.
Do women receive the same training, or is it different?
As mentioned above, not at all. Depending on if it is a male or female teacher, it might differ at the availability of receiving extra hints from any casual talking in the locker room.
I'm not looking for anything easy, gentle or "nice". As I'm sure there are also a plethora of websites out there offering this, do you have suggestions? Perhaps something on Youtube, as well?
I can't think of any right of the bat. But I can keep my eyes out. I interpret your "I'm not looking for anything easy, gentle or 'nice'" as it should be something that actually teaches how to "fight", pushes oneself both skill wise, physically and pain resistance. Some instructors can be quite humble or even goofy, but still delivers on making sure the students learn.
While doing a quick search I stumbled over this one. I have only watched the first 3 minutes, and it is boxing. But he goes through some good things, and if the rest is in the same style as those first 3 minutes it is worth to look through.
Do you still do this? If so, what is the most rewarding for you as far as "work-out" goes?
Rarely. My current focus is on learning how to dance (mainly Salsa, but starting to branch out) and Yoga. But if/when my schedule allows it, I will get back into trining either Bujinkan or Kickboxing.
Additionally.... do you believe that a story about this could be written to be believable?
What I have is this, so far: A slightly overweight woman has a life-altering experience. She would "transform" herself. Which brings me to inquire further --- say this woman is about 75 pounds overweight (I realize that this would be dependent on her regiment or schedule), how long would it take for her, or anyone else for that matter, to notice a difference? Let's say that her ultimate goal is to reach 115-120 pounds, and go from there.
Thanks in advance, as any and all help is greatly appreciated!
With losing 75 pounds overweight, if 120 pounds is a healthy goal, it might mean risk of getting loose skin. Younger people have more elastic skin than older, and quite a bit of other factors are at work. Having a slower weight loss gives the skin more time to adapt. I have heard numbers mentioned that having a weight loss of not greater than 2 pounds a week is preferable.
Second, training will lead to muscle growth. Muscles have a higher density than fat, so training might lead to a disconcerting feeling of wasting time as the scale isn't going down, while percentage of body fat would show a different story. A well trained woman can look quite skinny, until she uses her muscles, and unless having an extreme high testosterone count there is no risk of "bulking up."
For people to notice a difference, she and the people she hangs out with normally, would need more time to notice than someone she seldom meet. It might actually be more of noticing a piece of clothing being to large. Mental self image tend to change slow, and we humans tend to be quite worthless of noticing slow changes.
So say she is changing her life style (not just temporary changing diet) and pushes herself by going to kickboxing 2-3 times a week and start walking/jogging/running say twice a week, she is probably at least 20-30 pounds lighter in 6 month, and moves quite differently. As a book based on the US Marine Corps exercise mentioned, if it is comfortable it isn't exercise. Best effect is outside of the comfortable zone, but not to hard to so it doesn't lead to injuries, and enough rest for the body to build up.