First thing to check is the specs for the fan you want to use...it may have a limitation on how long the ductwork can be. Any good hvac place will have insulated round ductwork. If you can use Grainger, you can buy it from them as well. I use my vent during the summer, and not all that often in the winter, since I normally run the humdifier, so just opening the door means less the humidifer has to deal with. But, given it is on the roof, if there is a lot of snow, depending on the type of roof cap you use, it might be underneath, especially in Maine. It might be easier to run it to the gable end wall, but the length might be a problem. You don't want it to condense and then drip back into the bathroom. Hot, humid air will probably rise enough if you went through the roof to minimize that. If you had the run to the gable end rise all along the way, that would help the airflow as well. Dumping it into the soffit might work, but assuming you have vents there for airflow into the attic, you don't really want to dump all that moisture there.
See what other thoughts you get.