I am getting obsessed with walking and can't even sit for a minute I'm scared that I'll loose muscle

wait... how do you find the time to walk that much every day??

it takes me two hours and change to do 16k...

Since you're a recovering anorexic, talk to your care team. This may just be another manifestation of that.

Try a therapist that specializes in Cognitive behavioral therapy. Talk therapy that helps you recognize certain thought patterns and behaviors (their origins and frequency) and shows you steps on how to minimize/change how you react to them.

Your concern is a bit misguided. You should be more concerned about finding your way back home as you’re going to be about 20 miles away from home after you’re done walking.

Furthermore, there’s also the missing 6.5 hours from your work day.

But...
1. Could be unemployed?
2. Or walking as part of her working?

If you slip in a decimal and move it over it becomes 2.1 miles and 40-45 or so minutes. A bit more believable.

You might be spending a massive amount of time. How do you earn your living? I think that muscle will be secondary long term. Consider that this is probably part of your anorexia problem: a way of compulsion. Like the people with OCD that washes their hands, and in the absence of water they use wet wipes. You might be probably trying to progress on the food part, but probably the walking are your wet wipes.

I've seen a ton of people with anorexia on food treatment, that after that jumped into the gym and they do like 3-4 hours per day. Gym became their wet wipes. There are a lot of Workout benefits including walking, but this is not one in your case.

The muscle part is completely irrelevant. Be wary of this problem.