A TB20, carrying one person and not too much junk, will do over 155kt TAS for 8.5 hours, at 10,000ft.
That's just about from the south UK coast to Crete. I've verified this a few times, although not to the zero-fuel point, obviously
Others (some Mooney I believe) have long range tank options and will do over 2000nm; long enough from the US east coast to the Azores (though probably with tailwind).
The flying is easy - you sit there with the autopilot tracking the GPS. It just gets really boring if you are above an overcast layer and can't see anything.
The work in these "ensurance flights" is not the flying; it's the logistics. You spend a year organising corporate sponsorships, shipping avgas to far away places in drums, overflight and landing permits, survival gear, comms with base, you name it.
Nowadays, IFR is a must not because one could not fly it VFR but because the permits are for a specific date (or a couple of dates if you are lucky) and you can't just hang around waiting for weather.
I would guess that 68 years ago the hardest bit was navigation, followed by physical hardship in the cockpit. Not to mention the risk of engine failure in those Victorian-design machines.
Nowadays, for a crude record beating job in an SE plane, you would take a TBM700 or similar.