So you are making me think of doing marathon on huge.
However with the huge time investment...which civ do you go with. You will be spending so much time with one civ it better be an enjoyable one.
If your not usually a long game player i would try epic first. I played marathon huge in all civ games until a couple of years ago when i dropped down to epic. To be clear i am not dissing marathon games but be aware of some of the drawbacks of marathon games.
The start and end of the game can be a bit tiresome but the middle of the game is usually great.
The first 50 turns or so can really be a next-turn click-fest as things take a very long to build although you will likely have scounted out a huge portion of land before your decide if your going to focus war, general wide play or tall.
The end game can be a drag especially on a huge map and a very good gaming rig i think mostly due to the fact that civ 5 is restricted to the 32-bit system and it can take minutes between turns. That combined with the fact everything but war takes 3 times as long means i would give up a lot of games once they looked like it was obvious if i would win or lose.
Getting something wrong can be really, really bad. Losing units means it takes a long time to replace them, missing out on a wonder is huge as it can take 60-100 turns for a wonder to be built early game and one game i always remember which i basically lost in a single turn. It was late game and i had sea trade routes everywhere when i got DoW'd and all my trade routes got pillaged. I had no real stock of gold so couldn't even purchase trade routes and my income went into minus a couple hundred gold. I would take 10-20 turns to replace each trade route and my gold went negative in 2-3 turns so units started auto deleting, with a war to fight, so that was basically just game over.
There are advantages to marathon in that the AI generally suffers the same issues you do so if you kill their units it is much harder for them to replace them. If you have a really dominant UU you get a very long time to exploit it, you can massively cripple the AI economy etc as well as the fact that the above 'issues' can really make the game and events seem really historical and epic rather than realtively throwaway.
As for which civ to choose. In the current live version i would probably pick a good early faith civ as founding a religion easily makes or breaks a game at the moment and getting 250 turns into game to find out you missed a religion, especially when you were focusing on it is not a great experience.
Generally for marathon i would recommend as wide a game as possible as with lots of cities your less likely to spend numerous turns just clicking next turn while things build especially if playing peacefully.
I have settled on epic in recent years as it still takes me weeks to play a game but i find i actually finish more games while timescales still feel 'epic', era's and units don't feel inconsequential, things like wonders feel like a significant investment and when things go wrong it can still feel like a significant loss. I would also say that tall peaceful games are not really 'playable' on marathon where as all options are on the table in epic.
