Well I'm a 68 y/o male - not overweight - used to run a lot but what did my knees in was osteoarthritis. In 2022 I had both knees replaced.

Suggest you find a pt or trainer and get your leg in shape BEFORE the surgery, to the extent possible. After surgery, do the rehab exercises immediately and thoroughly.

As for post op pain, well it's a big surgery so there will be a lot of swelling and pain. The surgeon may give you a nerve block pump to partially ease the pain, and oxy for a few days followed by Tylenol. Ice and elevation of course, and wear a compression stocking on your foot and calf for a few weeks. After a few days, your inner thigh may turn black and blue from the tourniquet used during surgery.

Bet then, each day gets better. Range of motion improves. Most unhappy TKR patients don't do the required rehab/rehab and get scar tissue. Don't let that happen to you.