I could describe it as weakness.
Yesterday I was out shopping, some brisk walking, and sometimes it felt as if I were wearing my lower legs, as if they weren't really part of me.
If I walk a long way - miles - my legs give up.
It's not quite the same as being tired, it feels as if the instructions are no longer reaching the lower legs or they don't want to pay attention.
When first I was diagnosed with PN, the doc said that I should exercise the legs, toe stands... squats... etc.... just normal moving around would not be sufficient.
I think he was right.