They do help you, but they're just interval sessions really. As to the time for the marathon - well you need to be proficient at the distance to achieve that. (My downfall)
If you could do a long one as soon as you get off the ship, and just before you go back again, and then use the treadmill for shorter runs or intervals, then you should do all right. But it is the long run that is the key for Marathons. I presume that's what you are going for ?