So sorry OP. This is a long post, but info I dearly wish I'd know about a year ago. I really sympathise and could have written this exact post myself a year ago. At 40, I was told I'll need a knee replacement, but was advised to do what I could to hold off as long as possible. I too had put on weight due to immobility which obviously doesn't help.
The worst was when I was on a holiday and walking everywhere more than usual- museums, galleries etc. The pain was dreadful. Even in bed with my leg absolutely still, it would throb and ache. I literally strapped a hot water bottle around my knee, tried heated packs, adhesive heat things and analgesia. Nothing helped, and I was at the point of thinking an amputation would be better!
Do you have arthritis over the entire knee, or just half? Mine is mainly on the inner half and under the patella. Did they say a full knee replacement, a half or something else for you? As a minor, passing comment from 1 of the 3 orthopaedic surgeons I saw, he mentioned a specific off-loader type of knee brace. It moves my weight to the 'good' side of my knee and opens up the bone-on-bone part. I also had physio, but this was basically giving me a sheet of exercise pictures and telling me to do them at home!!!
The brace has meant that I can walk again, return to work and now lost 10kg! I did go on a keto diet, but wasn't absolutely strict, and feel the increased exercise was the main help. I'd read that every extra kg of weigh you carry, actually puts 4kg of pressure on the knee joint, so I was determined to get my weight down. I found swimming helped (when pools were open!)
My brace was provided by the hospital, but there are 2 main types I know of- but they will likely only be of any help if you have arthritis only in 1 half of your knee. The 1st is called a thusane action reliever off loader brace:
thuasne-store.co.uk/product/action-reliever/
I started at their largest size 6, but as I lost weight, it became lose and non-supportive. I then had size 5, but it too lost shape and became loose very quickly. Its also VERY expensive unless you can get one from the hospital. It is discreet though, and wasn't as obvious under trousers.
The next type I have is similar to the 1st one in this list:
thereviewal.com/best-unloader-knee-brace-for-osteoarthritis/
Its bulkier under clothes, but allows more range of tightening/losing as my weight fluctuates.
I'm now able to do 3hr walks with no pain at all and no analgesia (with the brace on).
If you have any questions, just ask 