Extend AC hose and install an additional fan
Since there is not any option to vent without extending the length of the vent hose I would try using a variable speed duct booster fan.


You want to be sure you are not restricting the flow of warm air at all. 
To help ensure this check the temperature of the vented air on a hot day before adding any length.
Then add your extension and booster.

Adjust the speed until the temperature of the exiting air is  the same or a bit cooler than original temp. 
Running faster speed than this may cause premature failure of the integrated fan so be careful. 

If your unit runs continuously this set up may be fine. If it cycles on and off you will need to think about how to get the booster fan to cycle with it.