I left in 2003 after 39 years. I think I should have left 3 years earlier!
Actually what do pilots do when they retire? They go flying of course! I bought an ex RAF aeroplane and still fly it. Trouble is I now pay to fly, not Her Majesty, but at least I fly when I want to, not when the RAF wants me to. And I only fly when the weather is nice!
Moreover, I am now far busier than when I was in paid service. There are millions of things one can do, some have been mentioned. For myself, I write, I am active in several Clubs, I fly, I am a Justice of the Peace (quite interesting that!). No pipe & slippers for me.
In all, I don't regret leaving. I had a ball, but time to move on.