How do single GA pilots prevent boredom during long flights?
Not long ago, I had my longest flight as a PPL: about 4 hours from Dallas, TX to Kansas City, MO, in a 172, facing a steady headwind the whole way, so making a ground speed of less than 90kts.

I ran checklists about twice as often as normal, and re-did my fuel calculations very regularly (in part because of the extremely slow progress). 

I listened to the ATC frequency, and tried to build a good mental picture of all the planes around me.

I went through every single setting and option on my MFD glass cockpit and iPad.  

I watched the scenery, both for visual navigation (despite my GPS), and just to admire the view.

Even after all this, the flight was pretty boring near the end, and I'm glad I don't do flights that long very often.