Yep......sometimes life just kicks you in the balls with no warning.Hopefully, you mean active mentally as well as physically? It would be a great time to get stuck into new interests... Learn a new language, or a musical instrument, write a book, whatever!
It is only 3 months until I officially retire, after years of scraping by in unofficial retirement. My ambition is the opposite of yours - to start a little computer puzzle games business to top up my pension so I have enough money to relax.
As for sedentary - ideally NO! I watched what happened to my parents over 20 years of sedentary retirement and declining health - it wasn't nice to see. I'd rather have 15 good years than 20 bad ones.
(Obviously, some people are unlucky and don't get a choice in how active they can be in later life. Make the most of what you've got, that must be the message.)
Make the best of what you have while you have it.