How do you politely ask a visitor to leave?

I am still recuperating and had a very active day yesterday. A friend came for a pre-arranged visit today. We sat on the deck and talked and it was lovely but she stayed and stayed- close to three hours. I was getting tired but didn’t want to be rude. Finally I said my tush was getting tired and I had to move.

Any tactful suggestions for how to handle this common situation?

Putting in General. Humor ok but snark will be flagged.