If I may...
It seems you are thinking that TKD is some sort of ultra intense cardio driven workout... one which requires beginners to jump and spin... possibly while getting kicked and punched.
Have never heard of such a thing. If you have been taking boxing lessons for awhile, I'm absolutely confident that you are already well ahead of the "normal" beginning TKD practioner concerning conditioning. I know you are well ahead of where I began.
I started TKD absolutely raw to physical athletic activity. I work as an artist, and I had been sitting behind a desk for the previous 6 years before I joined my dojang. I was probably 25lbs overweight and had zero conditioning. In my experience, and I would think this is quite common, my first month of training, was more about getting my body stretched out, learning some very basic kicks, punches and blocks, and then some gradual drills and bag work to begin some conditioning. A good instructor will get a clear sense of where you are physically and mentally, and bring you along at a gradual and methodical rate. My first pattern was after my first promotion... which in our school, a promotion is based on in class time requirements being met, and gaining our instructors confidence in being able to test and pass.
Of course, there were classes within my first couple months that pushed me hard in conditioning, but I always made it in fine shape. Sore for a couple days maybe, but a good sore. If you get yourself on a regular attendence, you'll be pleasantly surprised at how quickly you gain conditioning/fitness. I've only been training eight months, and while I feel very good about where I am right now in my training and conditioning, there are still more classes than not where I am totally spent at the end of the session. Several of those times have been from classes where all we did was practice patterns over and over. It's just like your boxing class... if you got on a heavy bag and punched nonstop, basic punches, it wouldnt be long before you were a wet noodle. I will go through the four patterns I know right now, and be breathing hard and sweating profusely. It's because of what I put into each and every movement.
So, if it's just a fitness/conditioning thing you are concerned about... IMO, it's not a real valid concern. You will be just fine. Go join, and go train. You will love it.