Well, when she asks you what you are thinking about, what are you thinking about?
If it's something about work, or about your son, or about a GAL activity, or about something you heard on the radio or saw on your drive to work--those are all things you can share with her.
If the answer is "stewing over whether you are having an affair"--no, don't say that.
But there's not reason not to talk about what you are thinking about if it's normal life stuff and not relationship stuff.
You build connection by listening when she talks, validating, expressing interest in what she is saying.
She's not expecting you to spend all your free time thinking about her problems. Those are her problems, not yours.
(Now, if there are joint problems--things having to do with son or finances or housework--where you should be helping to address the problem because it belongs to both of you, then you should be taking action.)
Me: 44 H: 44 Kids: 20, 16, 16, and 10 Together/Married: 22 years H announced he was emotionally detached and considering D: 4/4/16 H announced he is going to try to stay and reconnect: 5/1/16