50+ to lose, after knee replacement

Two things:

1. It's possible to lose weight entirely on the eating side of the equation. Of course exercise is good for a body, and lets us lose weight while eating a bit more and potentially getting a bit better overall nutrition, while losing at the same sensible rate.
2. I'd strongly advise not trying to lose weight during the acute phase of healing. Healing is job #1, and healing requires both calories and nutrition. Talk to your doctor about what's sensible and appropriate, but my suggestion would be to eat near maintenance calories for at least the first few weeks post-surgery. It's not a waste of time, you can still be learning about how to personalize your calorie goal, what foods keep you full, how to log in MFP most efficiently, things you can do to improve nutrition, and more.

I kept on the weight loss track after a relatively minor surgery that happened mid-weight-loss - laparoscopic gallbladder removal - and became sorry I did. Fortunately, I healed fine as far as I can tell, but I got weak and fatigued, then it took multiple weeks to recover back to normal. No one needs that.

You may even find that eating at estimated maintenance calories results in slow loss during healing: Healing burns extra calories above our normal needs. Honestly if loss seems to be fast in the first couple of weeks eating at theoretical maintenance calories, I'd eat a little more to slow it down. (I found I needed to eat quite a bit more right after a skull fracture/brain bleed during recovery a few months back. I'm still eating close to maintenance these few months down the road - bone healing is slow at my age - though my intention had been to creep my maintenance weight down a very few pounds. That can wait, in my world.)

I hope your doctor has referred you for post-surgery physical therapy. If so, talk to the therapist about what types of exercises would be reasonable for you, and at which stages of recovery. Those folks are a good resource for that kind of question, IME.

My friends - senior athletes among them - who've had knee or hip replacement surgery have been able to return to their normal workouts . . . in the long run, after a gradual re-build phase. If you haven't been very active previously, that same "build gradually" applies, post-surgically or even without surgery in the picture.

Bodies and circumstances are individual. Ask your doctor and physical therapist if there are specific activities or movement patterns you should avoid permanently.

I haven't had those surgeries myself, but have had other surgeries or injury-recovery phases to deal with, both before losing 50 pounds, during loss, and in the 9+ years maintaining a healthy weight since. Just for context, that period of time was 40s, 50s, 60s; I'm now 69. Some of the close friends who've gone through the specific knee/hip replacement have been active people - workout buddies of mine - in their 70s.

Best wishes for a speedy and smooth recovery, then the weight loss and fitness improvement goals!