I lost almost all my weight above 200 lbs only walking for exercise.
It's inevitable that you'll lose a little muscle mass even WITH exercise because it takes a lot more to carry around 315 lbs than it does to carry around a body that weighs a lot less.
That said, dieting makes you skinny in your clothes, exercise makes you look good naked

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If nothing else do some light cardio, like walking, and lift weights to keep your muscle tissues healthy as your body transitions. I wish I'd started lifting a long time ago.
And I'm ADDICTED to endorphins!!! My workout is my anti-anxiety/antidepressant prescription

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You can do this!