This is a question that you should discuss with the surgeon
who will be doing your specific surgery
, given that many types of surgery are more invasive and involved than others. In general, I would be very impressed if you are able to go canoeing one month after having open chest surgery, but it would, again, depend on your type of surgery as well as your recover and potential pain tolerance.
The diaphragm is one of the main muscles involved in breathing, and so in addition to being big, it is a vitally important muscle. Many people will require some time on a breathing machine after surgery to make sure that they are fully recovered and able to breathe on their own. Your surgeon will be have the experience necessary to answer all of these questions in the manner that is appropriate for you, but even the most experienced surgeons will have occasional complications or bumps along the road to recovery.
Anticipating that you will be able to return to full activity might be overly ambitious and make you feel like you are not recovering well, even if such is not the case. Please speak with your surgeon about this question.