I'm not familiar with "Chai" Tae Kwon Do, but I took Tae Kwon Do with my kids years ago. I was in pretty good shape at the time and it still pushed me. Our form was pretty much full contact and so it was nothing to take lightly. Many other forms are not so full contact and are much lower in intensity. Since it says, "Chai", I get the idea this is NOT a full contact form.
That being said, even in my class there were overweight folks there and they did fine. That should not be a big factor. We even had a guy in a wheel chair in our class. The teachers do what they can to accomidate all people in learning these forms. I would still say though, watch a couple sessions to get a good idea of how they run their class, then try a short term period before signing up for those year long contracts that can get pretty expensive. If you still think it is something you will enjoy, go for it. Keep the faith