So I’m on my longest flight yet (around 340km from the Shetland Isles to the Outer Hebrides (EGPB - EGPO). My Citation is happily flying along at 16,000 feet (and I do have AI on).
My general rule (bar training and activity flights) is to land and continue on from the same airfield before reaching the next one (a single journey) and not ‘skip’ to the landing.
What’s your approach to long flights? 
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Don’t do long flights. Don’t like sitting in a real plane for longer than an hour because it gets boring, so I keep my flights short and interesting. I once did a flight in the Arrow from Munich to Venice. But I kept low trying to following the valleys on visual like I were driving by car 
with varying degree of success I might add …
Stopped long flights many years ago. Like watching paint dry. Now almost all my flights are around 1 1/4 hours.
I think @Archer374’s advice is probably the best! If possible do flights in short hops. After all, you know the old saying, “Flying is the second most enjoyable feeling a person can have; and landing is the first”.
However, when I feel I must do a longer flight, I use Audible and listen to books. I think I’ve just about exhausted their (rather limited) supply of aviation material! (I can thoroughly recommend ‘Dangerous Lessons and Guardian Angels’ by J Spivack)
Takeoff, approach and landing are the zest of simming. So I keep my flights shorter than 90 minutes. More often than not half of that.
Last night I did Leeds/Bradford.
Up to the Highlands.
Then down the west coast of Scotland.
Landed in the isle of man.
Citation (same as you)
Had to climb to 40,000 foot, to get well above the clouds
I don’t do flights over 1.5 hours or 2 hours 30 at the very max. But to answer to you: I check the cockpit, scan for errors, check instruments, check the outside environment periodically, I try to remember what I learned in the airpilot book (respecting engine limitations, adequate engine cooling, fuel quantity estimates, adjusting for proper fuel burn, getting wind data and making sure to avoid storms and stuff and so on) then often prep a coffee, checking enroute charts and approach charts and visualizing the entire descent process and preparing for eventualities like go arounds or extended traffic etc…Just that kind of stuff…and since Msfs2020: enjoying the beautiful blue ball from higher up… edit: sometimes I have a podcast running in the background.
Yes I tend to agree with the posters above, that it’s more interesting to fly in short hops rather than long haul. The scenery in MSFS lends itself to lower, short flights as well.
Great answers. I share the point about short flights, I’ve touched down 76 times across the UK so far, but there comes a point where I’ll want to fly somewhere else and some somewheres are at a distance. 
How are you finding the citation? Thinking of giving it a try, been exclusively on the a320 so far
I have so far been doing flights from around 45mins to an hour and a half ish in a Cessna 152. I started at Shoreham Airport EGKA to Lydd in Kent and then from there crossed the English Channel in to France and so far have done a few legs and am around South Side of Central France. Done Limoges to Le Puy yesterday. I’m heading to the Med. From there who knows which way I will go with Spain or maybe Italy in sight.
Nice to watch the scenery change from Southern England and Northern France looking similar in terrain and weather to the Med like climate with more gentle winds and clouds. And will start to see more hills and mountains where headed to.
I also prefer shorter flights of 40mins or so.
I like preparing a lot of flight plans and landing at airports which shorter flights give me the opportunity to do.
a real dangerous question !
Sim rate x2 or 4 if you are doing cross country flights etc. But generally I do short hops of an hour or less.
The Citation Longitude is quickly becoming my favourite aircraft. It’s fast (shorter journey times) but you’re landing at around 150kn so forget little grass airstrips. 
I have to admit I like short flights that last between 1 hour to 2 hours. Oddly enough just yesterday I flew the Airbus FBW A320 from KJFK to EGLL London. That was a 6 hour 30 minute flight and I did not enjoy the flight. My approach to long flights will be I probably will stick to shorter flights from now on.
I usually go make a margarita and listen to music on my “in flight Mac book that I smuggled onboard” lol
I practice my Banjo playing.
I’ll agree with that. How people can do a 4 hour + . I know they aren’t sitting there, and probably falling asleep listening to the in laws chatter in the yard. So if they are doing that? Why bother having the computer running? I must be thick, I dont get it.
I guess read a book or watch TV, as long flights are pretty boring. I’d imagine in RL there would be constant scanning of the gauges, flight plan changes, preparing for next leg, paperwork, etc. - busy work. But in a sim it’s pretty boring and uneventful, I stopped doing them years ago. I stick to 1-2 hour flights, and rarely up to 3-4. But I also don’t accelerate time or use AI stuff…