I had five bypasses 23 years ago at age 48. Strong family history of heart attack deaths, so my genetics are stacked against me, but I was inactive and obese. First the good news, you get to sleep through the whole thing.
Number one tip: They will give you a fat round pillow post-surgery. You will come to cherish it. Hug it tightly before you sneeze or cough. If they don't give you one, get one.
Number two tip: Change your lifestyle. I began a low-fat diet and lost 65 pounds before medical science decided that you can't lose weight that way. So glad I didn't know it. Went from 300 to 245, which is not fat at 6'8". Hospital gave me an exercise progression. First step was shuffling my feet around the den for five minutes, then ten, then 15 around the front yard. Then to a local college track where I progressed to five miles, then began jogging. Stress-cracked a foot then broke a metatarsal at age 54. Desperate for exercise, began using wife's five-pound dumbells. Buying more weight as needed, I progressed until I have dumbells up to 75 pounds in five-pound increments. Bought a Bowflex, used it hard for several years, ignoring Bowflex routines and doing a heavy-duty regimen. (It is a great piece of home equipment, despite what those who never used one will tell you.)
Weight went to 265 with no increase in waist size. Seven years after surgery, my cardiologist said he was "stunned", that I had the heart of man who had never had any heart problems.
Now nearing 71, other health issues ( lifelong back problems, first back surgery at age 22) have forced me into inactivity. I have had five stents over the last four years. I have not gained any weight but some muscle has been replaced by fat and it is driving me nuts. I had become thoroughly addicted to exercise and miss it, after back surgery in April I am trying to become more active.
Hope everything goes well with you.