@CatOnTheCarpet they're legitimate concerns, but they're easy to deal with!
Re intellectual/mental stimulation, stuff like having time to read the paper each day (as opposed to just consuming news through headlines/listening to the radio/watching TV) is good - i.e. you have the time to read more detailed analyses of stories.
Crosswords/sudoku etc are also good - and have been cited as important aids in helping with/deferring ailments like dementia see this piece here
I've been learning Mandarin for some time now, but actually have the mental space as well as much more available time, to focus on it - which is also good for my brain.
I wondered if we would be bored, but so far that hasn't happened! DH and I do make the effort to go out and do different stuff - things we never had time for when we were working, e.g. a visit to an exhibition, a walk along the canal, going out for lunch etc.
We also have a few friends who are retired, and spend time with them - it's great to be able to plan things during the week, rather than having to have everything happen at the weekend! We've done a few of these Mystery Guides with them, including a recent trip/overnight stay in Edinburgh.
DH goes out running 6 days out of 7, I go to the gym/for a swim. Off-peak membership is also cheaper.
In other words - there's plenty that will occupy you!