At the moment you are at a high risk of losing muscle based on what you are eating and the amount of walking you are doing.
If I have this right your TDEE is around 2700 calories so even if your evening meal is 800 calories (which would be a hell of a stir fry!) you are running a massive calorie deficit. You could lose weight pretty quickly doing that but as you don’t have much to lose to hit a normal BMI (about 10 lbs or so) a lot of that would be muscle loss.
I’d suggest upping your calories to at least 2700 if you want to start training and focus on building the muscle mass/strength.
Again, @GTIPuG will know better than me but I was under the impression a 5x5 will be more geared towards building power than mass/strength. For that I think you’d be looking more at a 3x10-12 program.
It is possible to simultaneously lose fat and gain muscle but only in very specific circumstances (generally untrained, older obese individuals) which don’t really apply in your case.
So I’d be looking at eating at TDEE or a slight surplus, making sure macros were right (especially grams of protein per lb of body weight) and following a training program to build mass. Once you have that down it’s a far simpler proposition to have a controlled period of deficit to drop any excess fat that’s still hanging around.