A century or an endurance ride is any ride that the rider finishes without changing into regular clothing. As long as you haven't stopped for a shower and gotten changed into "non-cycling attire" then it is considered as 1 ride.
The reason for this is that some long distance events like Brevets 400-1200km will mean that the rider might stop and sleep briefly, possibly shower but then put the riding gear back on and continue.

Either way it is all too far for me.

I'm still building up the courage to do my first double metric/125mile ride in October. Thankfully the start/finish is only 15km from home and is around my local roads.

CHEERS.

Mark