Dinkee,
I have a feeling that that walking is probably elevating your heart rate a bit more (and is burning more calories) than Tai Chi - however, it seems like a great idea to incorporate both into your fitness regimen. If you find that you are working hard during the Tai Chi sessions, you could certainly consider alternating between the two. Given that you are biking and swimming on other days of the week, I imagine that choosing one or the other won't really impact your fitness (or weight) bottom line. I think the most important thing is that you are keeping it interesting and engaging so that you are more likely to stick with it long term - nice job!
Take care,
Dr. Beckerman
on Twitter @jamesbeckerman
I personally would do the walk in addition to the Tai Chi class. However, on the days you do Tai Chi I would leave the weight behind, at least initially till you become accustomed to the new exercise. Tai Chi can be a work out in itself and I think you should listen to your body. If you feel tired, either go for a shorter walk or drop it entirely.
It will be interesting to see what the doctor recommends.
Well done on the weight loss and behavioral changes.
J