I, too, am much in the camp of working on your long term lifestyle rather than a short term weight goal.
When I started having to do that six month insurance thing, I looked at what should my diet, and overall lifestyle, be in five or ten years - not six months from now. Basically what an RD would tell anyone they should do to live and feel better and longer - food pyramid, my plate, etc. all represent a basic healthy diet of leaner meats, more fruits and veg, whole grains instead of the overly refined/processed white flour stuff, minimize sugars and in particular - junk foods (high calorie, low nutrition) whether they be high carb or high fat - or frequently both. It was not perfect to any one particular standard, but it was what I could do, and thus, more sustainable than any "book" diet. It was also an evolution (and still is) as I worked to drive my preferences in the right direction.
I also started with going to the gym (local Y) and got back into swimming, which I had always liked before, and found I liked playing with the weights. That was fifteen or so years ago, and I am still at it, though in a somewhat evolved form (particularly during Covid!). But the lesson here, much as with diet, is to make it something that you enjoy (or at least tolerate) and make it a habit. It can certainly evolve, as you say with starting out with just walking. Carve out the time to do something, and let the activity evolve with your ability, interests and circimstances.
I had no particular weight goal in mind for this phase, but as I noted, it was an exercise in developing better habits for the long term. As it turned out, I dropped around 50 lb, or about a third of my excess, in that six months which caused a rethink on the surgery idea, at least for a while - but that is another story. I suspect that I would not have done as well if I had been aiming fora specific pre op weight goal.