Knee surgery coming up !

Been there, done that. I had meniscus surgery 2 years ago and did pretty well, weight wise. Maybe gained about 5lbs. I also had 2 other surgeries in the course of the same year and with the second one (a cancer surgery) I gained 20lbs! It was hard several month recovery and the stress factor didn't help. Lots of tv and junk food binging. I did work very hard once I got my mobility back to lose that weight and so when I had the 3rd surgery I had a plan in place and didn't gain a single pound...even lost some! That said...the recovery from meniscus surgery isn't bad or terribly long.
My advice is to meal plan and log your food religiously! Cut your calories down to reflect your activity level. During my 3rd surgery I went to 1000k/day for a week (under my doctor's suggestion), becuase I was laying around all day, only getting up to use the facilities and was not permitted to lift anything heavier than a fork or remote. You won't need to go that drastic with meniscus surgery. Before that surgery I pre-made a bunch of healthy meals and froze them in portions. I made sure there were no crappy snacks around the house and also ate smoothies daily (helps with bowel motility since anesthesia pretty much locks me down for a week).
Remember that during surgery they give you lots of fluids, so don't panic when you jump on the scale and see a dramatic increase! I weighed myself when I got home from the 3rd surgery and had gained a whopping 9lbs in fluids that took me 8 days to shed.
As soon as you get the green light to be active, jump on it! Even if it means just walking around the block. Move as much as you can and do whatever your doctor permits. This might mean changing up your workout routine. I.E. If you were a runner, you may need to walk, bike or swim for a few weeks. You may not love it, but it'll burn calories without damaging your repair. I believe I was back to my normal routine within 6 weeks after meniscus surgery.
Good luck!