Well, I'm currently down 40kg aged 49.
Exercise is far more useful than it's generally given credit for - it increases your muscle mass, means you use more calories just to exist and enables you to still eat well. It also makes you feel better - any emotions that could send you to eat can be deflected into digging in that little bit extra, giving you a release, relaxation, the feeling of satisfaction, better sleep so feeling less tired and off colour, etc.
The key for me was combining exercise (mixture of gym/weights, rowing and plodding around the park, mostly) with something that sounds like a fad - I got diagnosed with Celiac. So going gluten free.
Now, cutting out wheat, barley, gluten and oats sounds stupid and it was purely on the basis of medical advice for me, but what it actually represents is eliminating the vast majority of prepacked, highly processed, high carbohydrate, high fat and low protein/nutrition foods.
Because you can't think 'sod it, I can't be arsed' and get a cheese half pounder with mayonnaise, fries, onion rings, 2l bottle of Fanta and a box of Cornettos because the diet's screwed tonight, I'll start again tomorrow - or pick up the appealingly packaged stuff in the supermarket - you've got to plan what you're going to eat, so you have a chance to make choices rather than impulse buys. And it's your choices that makes for a longterm habit change, not 'I must not eat/have to go without because I'm fat and my body is now permanently fated to put as much weight on as possible'.
The sort of things we have include -
- chicken thighs, roast veg in olive oil and rice,
- slow cooker chilli with black and kidney beans, extra veg on sweet potatoes,
- Chilli salmon with rice, edamame beans, rice, tomatoes, cucumber and grapes for dessert,
- GF pasta with a mini pot of cream cheese, petits pois and smoked bacon bits,
- egg & bacon on avocado GF toast,
- homemade blue cheese burgers with mushrooms, all the usual salad items in a burger (no bun) and homemade wedges,
- Chickpea, spinach and sweet potato curry
- sausage, egg and wedges,
- steak, salad and potatoes,
- pork casserole with tomatoes, aubergine, peppers, courgette, chilli, garlic, onion, capers and olives,
- Roast chicken with 4 types of steamed veg and herby roast potatoes
- whatever meat is on offer with salad and rice or potatoes
- so not exactly half a lettuce leaf and a sniff of a dairylea triangle once a day.
I can't do breakfast, so I have a good lunch (usually fish, rice, salad, but can be pecorino, apple and GF bread with garlic oil or a GF cheese and salami sandwich) and an emergency 'I must eat' can be things like yoghurt, dates and pistachios or a mini brie, hard boiled egg, tin of mackerel and olives; stuff that's nutritious but most importantly, that I really like.
What I would suggest as you've got knee pain, is check your gait (at a running shop - does your leg turn in or your ankle drop inwards as you walk, for example?) and get a good pair of running shoes with some orthotics. That'll ease some of the pressure on your joints, meaning you'll be able to move more, be less tired from the effort, less likely to collapse on the sofa and will enable you to do things to strengthen your leg, back and core muscles.
Walking can be good, but if it causes you pain, going to a very cheap gym and learning to use the rower and weights machines also provides very useful exercise and strengthening - then you can step up the weightbearing exercise when you are stronger and more able to handle your own weight. Swimming is also good, as long as you plan for being starving when you come out and have a good snack in your bag to keep you away from the chocolate and Red Bull or the chip shop.
In short, though, yes, it can be done. Without subscribing to fancy branded plans, without making particular writers a fuckton of money, without thinking you've been promised the easy solution and answer to all your problems by some God like being.
If you are also found to be a suitable candidate for Ozempic/etc, that may help you, but it's not a foregone conclusion you need that, surgery or anything else; you can change if you are ready to.