On and off, we kept chipping away at our task, sometimes meeting every few weeks for a couple of hours, sometimes once every couple of months. It was folly, but by the time we realised, it was too late we had become thoroughly invested, the game had taken on a life of its own, and there was nothing else but to see it to its (eventual) end. If we had, we'd probably have played something else instead.
We must have embarked on our epic enterprise sometime in late 2013, though neither of us suspected we were about to spend the next five years embroiled in a seemingly never-ending coop hot-seat game of Civilization 5.
For the first 5000 years, nothing much happened.