Take it easy, listen to your instructor, and you should be ok.
Different teachers most likely will do the classes differently, so what I learned may not be exactly what you will be learning, although there will be similarities and overlaps--not only in techniques, but even calisthenics prior to the actual lessons.
My experience with FMA was that it was not as aerobic intensive as martial arts where you are constantly jumping up and down, and kicking such as tae kwon do. The large muscles of your legs require greater exertion than the smaller muscles of your arms. The fact that you may be overweight would be less of an issue here, and if you continue with the lessons, you'll probably lose weight just because you will be moving.
What do I love about FMA?
First, it is a cultural thing. I was born in the Philippines, and although I reside in the U.S., it is a way for me to touch my roots in a much, much deeper way than just eating Filipino food, or listening to Filipino music.
Second, FMA in my experience is...devoid of much of the fluff that is associated with some martial arts. I won't name any ones specifically, but the major emphasis in FMA is to practice with a
live partner. We never studied "katas" in my school, although they may exist in others.
Third, FMA is efficient. Long ago, before I studied FMA and JKD, a fellow kung fu student of mine went and studied FMA taught by Dan Inosanto. After he came back, our kung fu instructor, being curious asked him to show him some moves after class. To empasize my point, our kung fu teacher had studied Shaolin Wu Shu for a number of years, and had specialized in the six foot staff. My friend, on the other hand, only completed a
one month seminar in FMA...
I watched from the sidelines, seeing both men, one with a pair of rattan sticks, the other with the pole square off. Now mind you, this was not a fight to the death, or anything like that. It was just two interested martial artists sparring slowly with dissimilar weapons.
What I noticed was when my kung fu teacher would block an overhead blow with his pole, my friend would use that opportunity to lightly rap the teacher's hands with the rattan stick. Not hard at all, but just enough that it was clear that a real blow could have broken, or at least rendered the teacher's hands useless. This message was not lost on my teacher, and I remember him just smiling and shaking his head. One can only imagine what he was thinking.
Useless hands cannot hold a weapon. This concept is called
defanging the snake.
It has been said of FMA that much of its history was that of an art passed down from one family member to another, such as from father to son, or uncle to nephew. There were no large "schools" such as one sees in Chinese or Japanese martial arts tradition. Therefore, in a sense, the FMA one learns today is the result of a natural selection process. If a father or uncle, for example, had died applying his FMA in combat, or in self defense, neither would have survived long enough to pass on the tradition.
Some FMA schools place more emphasis on stick training, others, such as Sayoc Kali, place more emphasis on blade training. IMO, it's all good.
I hope you enjoy your experience. Let us know how you do and what you think.