The Marathon has a very thick tread on it as compared to most 700c tires, and while I'm not privvy to the chemical content of the rubber, I'm sure that it's probably made to wear longer as well. While I am not familiar with Duranos, it's not surprising that the treadlife is less with a lighter, easier rolling tire. Those are the tradeoffs that you make. If you want a long lasting 700Cx25 tire, try the Gatorskin.
If you think 2000 miles is bad, try something like the Grand Bois Cypress. It's a very light, thin 32mm tire. I barely even get 1000 miles out of the rear tire.