Retirement doesn't have to mean swapping responsibility and mental challenge for nothing.
Nor to vegetate, watch day-time TV, and die like Blue Matt's customers.
There are plenty of opportunities for responsibility - political or community activism, volunteering, and caring for family or friends. I know lots of retirees with more responsibility now than when they worked - mainly through charity work and community projects.
It is also a chance to read, listen and watch all those things you missed when you were working.
I early retired over 7 years ago and spend a lot of my time walking in the Pennines, on UK national trails (well over 1000 miles so far) and exploring this country and beyond. My knees are in a worse state than when i retired but otherwise I'm fitter.
It should be an opportunity, not a threat. Seize the day!