Yes, this is an expression in English. It means as comfortable as possible. The idea is that old shoes are broken in. They won't pinch your feet and they won't cause blisters like new shoes might. I hope this helps.

It's a metaphor, and in my opinion not a very good one haha. The author is describing the relationship between his knowledge of English and worn shoes because that person's knowledge of English will be second hand like putting on shoes or tying shoes (not having to think about it). This might be a tad off but its closely around what the author is trying to get at I believe.