My legs were broken down

[output]

Is the following sentence correct and natural?

My legs were broken down with kept walking more than 10 hours a day.

 

[tedmc]

We don't say legs or body parts break down; we say machines break down.
You could say:
I couldn't walk anymore after continuously walking for more than 10 hours a day.

 

[emsr2d2]

I think tedmc has got to the crux of what you're trying to say but your sentence doesn't make sense and it isn't grammatical.

My legs were broken down
My legs hurt/My legs were painful

with kept walking
because I had been walking
after walking

I think you need to give us some more context. Why had you been walking for more than ten hours a day? For how many days had you been doing that?

 

[Tdol]

Or My legs ached/were sore