Marathon is for people who don't understand how to play the game. Each game has the same number of decisions regardless of the number of turns - increasing the amount of time it takes to build a unit or improvement doesn't improve the game play.
While I think this statement isn't true, actually I would prefer if epic and marathon would only increase research time and not building time.
I like to play in ancient-medieval age. Or even until rifle, but not more. But that doesn't mean I like to produce less units or buildings, I just want the older ages to last longer and let me do more things.
I play epic, and I would play marathon for the longer tech time, but things just take so damn long to build too.