At least there is plenty of daylight up here in June.
..and every minute of that daylight my be needed for anyone attempting this on a small wheeled bike!
Sixty miles on these roads/tracks will be a challenge on anything older and certainly anything heavier than needs be.
I would probably err on the side of a light steed, bearing in mind not much extra weight need be hauled thanks to transport for camping gear, so the bike needs to cope with not much more than rider weight.
Thinking cyclo-cross frame built up with some 'comfortable' components.
No need for mudguards, but they would be aesthetically pleasing.
Agreed fattest tyres possible for comfort.
All power to those fit enough to céilidh at the end of such a ride. In my experience, the participants in these type of events usually end up staggering about without the aid of whisky!
The doctor diagnosed me with hereditary diarrhoea. I can't understand, I wear a kilt.