My thoughts, for what they're worth...
Generally, the better shape you are in, the faster your recovery is. Especially for the first surgery, I walked a great deal before I went in. This second time, my symptoms were much worse, and I wasn't able to do as much beforehand. Youth is obviously a big friend to you for the surgery, speeding your healing and the return of your energy.
The exception is your rib cage. If your sternum is cut, it's at least four weeks for it to mend, and a few more weeks for you to bring it back to its fully carrying and flexibility capacity. Trying to exercise it in any weight-bearing way before the bone has fully healed can delay or even derail recovery, as the bone bond will weaken rather than strengthen (similar to flexing a piece of metal until it becomes weak, heats up, and snaps apart).
Careful upper body stretching/twisting is good, to help return flexibility and work out some of the soreness from the chest muscles, as long as you don't flail wildly or become too macho about it. Again, the issue of moving forward vs. damaging progress your body is making. You have to extend your flexibility in increments, not try to get it all back right away. You should be walking every day, with the distance and speed based on how you are feeling that day, rather than a printed schedule.
Some tips for walking:
- Start out slower, so you are sure to have energy for the return trip (you can speed up in the home stretch if you have energy to burn).
- Hit the bathroom first - they are rare out on the suburban roads.
- Walk to a place that has a halfway stopping point, if possible (like walking into town). That way you can rest before you begin your return, if you overestimated your energy that day.
- Take a cell phone and have someone you can call, if you get stuck or overtired. Don't be too proud to sit down on the curb or someone's steps, if you need to.
- If you're not feeling it that day, do short walks that day. You'll more than make up for it over time.
I am 57, and was out of the hospital three days after my first surgery (at 52) and again three days after my second surgery (three weeks ago). Both times, by the second week I was walking five miles in a day without pushing myself (very important to listen to your body on this - some days you can, some days you can't). Both times I initially lost weight (mostly from a temporary loss/warping of my sense of taste), and the first time put most of it back on over time. I have yet to see if I will put it back on this time (betcha I will).
Basically, I subscribe to the "move it or lose it" theory of recovery. However, I add the corollary, "abuse it and it will abuse you right back." The tiny muscles (intercostals) in your rib cage can completely immobilize you in pain, if you overagitate them and they spasm. They're already irritated from their positioning during surgery. They can actually make you feel like you are having a heart attack, like a giant hand is holding you down and keeping you from drawing a full breath. So, regain your flexibility in increments.
At your age, you may well be feeling fully physically competent after two months. Your body should continue to add to your capacity for about a year after surgery, as it adjusts the size of your ventricle and realigns itself with its new plumbing.
Best wishes,