You've gotten some good advice and some of questionable value. To restate what I consider to be good advice:
Go wherever you please, but go somewhere. Do not expect to be Mr. Universe in a month, nor a year. Just keep at it. Most likely, you will find every time you get better at strength and stamina, you will realize how much better you want to do, and that exercise goals aren't bad, just slow in achievement. But you can get there. There are reasons for improving strength and stamina. It will also make you faster and able to punch or kick harder if your chosen art needs that.
Don't worry about stretching or high kicking. You may never be able to do that, or you may do it inside 2 or 3 years. At least you don't want to lose what you have now. Talk to any instructor about your self-perceived limitations. They should be willing to help you improve.
Listen to other student's advice as well, but don't let it get you hurt of frustrated. There will come a time when you will want to give advice to new students. Understand you are only being told what worked for the person that gave the advice. It may not work for you, but don't be afraid to at least try. Remember that when you decide to give advice.
Understand that belts are a visible sign of where you are in your martial arts journey. To the extent that your fellow students and teachers help you develop and learn, gaining belts is a sign they didn't waste their time. Gaining higher belts should not be a goal for themselves, but they should be a goal in that they indicate progress. So I don't recommend that you put them down to quickly. Your fellow students may not understand.
Do quit smoking! Don't get discouraged, nor use the fact you may get discouraged as an excuse. I think I must have quit about six times before I really quit. That was about 40 years ago. I could not quit 'cold turkey.' I had to smoke cigars (I am physically unable to inhale cigars) for a month or two to have a chance. Some times it was more than a couple of months. But I would encourage anyone to try nicotine gum first. It seems to help a lot of people.
Don't be too quick to choose a martial art. Look around. Any properly taught martial art is a good one. It will improve your strength and stamina, and improve your ability to defend yourself.
Good luck. Let us know what your chose and how you improve.