Not so mid life crisis

As someone said upthread, local libraries are usually a good source of activities - book groups, past times chat groups and local history seem to be very popular; Drop in and Draw (materials provided - no drawing experience needed!).

If you have any walking groups near you they are a good informal source of exercise and company and cater for all fitness levels.

I give a few hours of my time two days a week listening to children read in a local junior school. Schools are crying out for reading volunteers and it’s very rewarding.

I’m sure you’ll settle into this new stage of your life soon and find something enjoyable and rewarding to fill some of your time. Don’t rush into anything.

I'm still volunteering but on my own terms .

Oops lost the first part of my post .
Just a warning about volunteering.
It's not always that great .
I've never encountered such a rude ,unpleasant and nasty group of women

I joined my local u3a and have met a lot of very nice people as well as started a few other interesting activities. There is often a lot to choose from . Have a look at u3a online for your area .
If you like to walk there are usually walking groups with u3a, also many councils organise regular free walks locally too.
My dh joined bowls when he retired and over 20 years later still enjoys it.
Congratulations on retiring , you may find that you are so busy you wonder how you fitted work in.