Although I have no statistical proof, I have a hunch that most players on these boards, myself included, would consider marathon to be easier than normal or normal, simply because the whip and the chop is so much more powerful. That being said though, if you don't take full advantage of the cheap units and go for a peace mongering victory, it can be pretty rough.
Was playing as Louis on monarch on a huge map going for a culture win without DOWing on anybody and ended up being dogpiled on by Charlie and Wang. The main problem was that my buffer and ally, Gilgy who normally does strong got plowed by Charlie early enough for Charlie to set up along my border and join Wang for the final war once my second city hit legendary. Although I had enough units to deal with Wang's stack of cavs, rifles, and arties, I had not counted that gilgy would get rolled by Charlie in 30ish turns when Gilgy had kept Charlie at bay for 5500 years. My last city had nine turns left but unfortunately it was only six tiles away from Charlies SOD which was huge.
The moral of the story is had it been normal speed, I would have won before Charlie finished his war with gilgy, and if he had won earlier, his stack wouldn't have reached my legendary city in time to stop it. The main thing though is the marathon speed allowed the AI to make greater use of the tech lead they gained on me after I switched to 100% culture. So only 5 turns after Wang teched Artillery, my cannons and cav were suddenly going up against 20 or so artillery in addition to the rifles, cav, and airships Wang brought along.
If you don't take advantage of the many bonuses of Marathon, i.e. pursue ancient-classical warfare, it's not as broken as people think it is. Yes you will get a better start and earlier finish dates on marathon then on normal but pursuing a peaceful victory on marathon can have it's own little quarks and difficulties that normally wouldn't be a problem. I wouldn't say Marathon is completely and utterly broken compared to normal, it's just that it's degree of brokeness depends on how you ultimately decide to play the game.
