I recommend buying Voltaren gel if you expect these occasions to recur. I do not know if it is OTC in Britain; it is in the US.

It is a topical NSAID. Much safer than taking oral NSAIDs that can wreak havoc (GI issues).

In the past I had foot stress fractures twice from suddenly walking for a very long time, much longer than usual, and in my case it was also in wrong shoes. Maybe with the right shoes it is not an issue. But now, if I walk much more than usual, I rub Voltaren gel into my feet before going to bed and they are fine next morning.

I do not think such long walks are a problem if they are regular and your feet are used to it, but when it is a sudden change, it might be hard on your feet.