This tendency to "boredom" has been exacerbated by the people spending a lot of time watching TV and other media, as well as instant gratification culture due to technology.  If I want to watch a movie, I rent it and start watching immediately.  I don't have to wait until the time it's scheduled to play on some channel or even go out to the video store to rent it.  I can send an e-mail or text to someone instantly and don't have to find time to call them (or when they can take my call), or send a letter in the mail, which could take a couple days or so.  Between the constant stimulation of TV, music, etc. and everything being available instantly, when there's a lull in the "noise", the mind becomes uncomfortable with the silence and inactivity.  I remember as a child that we didn't have much to do, say, in the Summer time, but I'd just go out in the back yard and look at things ... trees, flowers, various thing in nature (including squirrels, bugs, birds, etc.)  Maybe I'd go exploring or go for a walk.  I remember that I could just quietly look at things and be content.  That's one reason the silence of the seminary was easy for me, since it came naturally to me.  But if you constantly have your mind buzzing and occupied and active, when the activity ceases it makes these types uncomfortable and restless.  I actually pity people like that.

I’ve never understood when people complain of being bored.  If you have a mind and imagination, how does that happen?  Even if you can’t be physically active, doing something, you can always engage your mind’s eye.  

Would you say boredom is primarily in the intellect or the emotion?  

There was no such thing as boredom in my family.  Complainers were cured by being assigned a chore, clean the restroom, the basement, the attic, the garage, mow the lawn, weed the garden, wash the car inside and out then polish, etc.  

I sure fit the description of a dull mind.

What would be some intelligent and legitimate ways to engage your mind, apart from work?

Plan an art or building project, plan a road trip, pray, recite poetry, quiz yourself on various subjects, write a story or poem, write a song, sing songs, plan a family or friends gathering, plan a menu, revisit a time or place from the past, the list is endless!   All can be done in your mind, in your imagination, no? 
If you can’t do this, you need to train your mind.  (What is your age?  Are you Gen Z or Alpha?  Children arrive at school without imagination.  I started seeing this in about 2014 or so.). So sad. But it’s reversible.  The younger, the better.)

I am in my mid-thirties.

I do have an imagination, but I have read some spiritual writers say that daydreaming is a bad thing. It is difficult to draw the line between a vivid mind and a dreamy mindset.

Deliberate, focussed, purposeful use of the imagination is anything but daydreaming!