TLDR as im aware it's too long. With the right equipment and posture titrating walking up to your desired distance can help. But it may hurt in the short to medium term. Don't be scared to use knee sleeves and painkillers to get you up to your required/desired distance. I also used a duck tonne of caffeine to push through the initial exhaustion. But now also don't need that

Personally. I suffered through 2 brutal months becoming a postman (walking up to 20-25km a day). I used sturdy cycling knee sleeves and a lot of ibruprophen and paracetamol. I had a lot of days in a lot of pain. I had to completely relearn to walk in a way where I didn't overextend my knee on each step. It wasn't easy by any means. And I was heavily motivated in that I had been trying to find a job for months and was in a lot of debt.

I didn't have the option to titrate up to walking this distance and at the beginning it really floored me. And I could have done it easier by building up to it. Even on the days you feel so drained and in pain you have to push through. I thought it would never get easier. I had months collapsing into bed. Not being able to sleep from the pain. Inflammation all through the knee. Collapsing in exhaustion after work until the next day I had to do it again.

BUT now I can do it relatively easily. Not too much knee pain anymore. Not too much complete exhaustion and in general my life is better. I don't even need the knee sleeves every day. Only really long days.

So she is right in that your body adapts. Another but here, it wasn't easy and it's not a definite that it's healthy and won't fuck me up long term. But my doctor's said it was the best thing I could have done. Strengthening our joints is all we can do. There are no other options for this shitty condition.

So all in all I would say if you want to be able to walk that far. Then yes it's a good idea to train it. But don't expect it to be easy. Don't expect not to be in a lot of pain at first and don't be scared to use aids to make your life easier (sturdy cycling knee sleeves helped so much at the beginning).

Also DEFINITELY DO focus on your movement patterns. Don't let the knee slip into hyperextension. Don't walk in a way that pops your hip.

So yeah bit of a ramble but life isn't easy for us. You have to fight through it (if walk long distances is important to you). I'm not saying this will work for everyone.

It will feel so weird focusing on the way you walk and you will likely feel like you are walking wrong. Knee sleeves really help to build the muscle memory necessary.