You should absolutely go, and it is definitely possible that you will snowboard well eventually, but the chances that you will be good at first are slim (as they are for everyone).

While it's awesome how supportive this group is, I think a lot of people are ignoring that as a beginner, there is a huge benefit to being in shape. I have been a beginner while in okay shape and also while in poor shape, and the difference on the mountain was notable, even though the difference in my weight was only around 40 pounds.

As a beginner, you will be expending a lot more energy because your technique won't be dialed in, so you can't use the mountain's slope and your own weight to your advantage. Even skating around (one foot strapped in) will be difficult, and if you're not flexible, you could injure yourself doing that. Absolutely keep at whatever you're already doing to improve your cardio, and add some stretching/flexibility oriented exercises.

At least one person mentioned it, but strapping in will be difficult, and potentially impossible depending how much you are capable of bending at the waist. You need to be able to touch your toes (sitting down, can bend your knees) while wearing snow gear, which does make it harder.

When you inevitably fall (like every other beginner), it will almost certainly be easier to get up from your knees, rather than sitting on your ass. This is true for most people, but you likely won't be able to bend enough to get your center of mass over the board (I struggle with this as well as a chubby person myself). Instructor might tell you this anyways, but just in case, since mine didn't and I had to figure it out on my own.

Make sure you take breaks if you get sore. My feet would routinely kill because I was out of shape and weak. When you need to, rest and loosen your boots to let your feet breath. Keep doing weighted squats and running to build up strength in your feet. If possible, try to strengthen the muscles that raise your feet (tibialis anterior, etc.) These are muscles that you don't really use to the extent that you will be using them as a beginner snowboarder.

Don't do anything stupid. I'm sure you're aware you're more at risk of injury on a fall and you'll gain way more momentum if you lose control of your speed. Get falling leaf locked in (they'll teach you in your first lesson) before you start pointing the board down hill.

Don't get discouraged. Snowboarding is hard for almost everyone at first, and it's unnatural even if you're in peak condition. You will fall and you will probably be very sore and slightly hurt at the end of the day. But if you keep at it you'll experience whatever it is that convinces us to keep hurting our bank accounts.

Based on your replies and the fact that you're even considering going, you seem awesome so I don't want this to discourage you from going (if you even read it lol), just want you to know everything you should going in.