As far as IBA rides go:
Discipline and mental strength.
You can plan the crap out of a ride.
You can watch your diet in the days leading up to a ride.
You can watch what you eat on the ride.
You can be as fit as you like.
But if you can't keep your arse on the seat for the desired time and distance (whatever that may be - 24 ,36 or 50 hours) then all of the above is totally futile and it's game over before you even start.
Discipline, to keep to a rough plan.
Mental strenght, to not talk yourself out of it if your sore or ache or tired. BUT it's important know the difference between tiredness and fatigue and when to pull the pin.
IMHO.
P.S. There's no rocket science involved in LD riding........only opinions
P.P.S. As far as casual LD riding goes.......whatever feels confortable.....