I know what I would do but that doesn't mean it would be right for you. Sounds like your a stronger paddler and can take advantage of a longer boats speed potential. You don't plan on carrying 75 lbs of gear so you don't need a big touring boat. So I would be looking for faster boat which means it is more efficient. That way you get more distance for the same amount of effort. Or you could say you would put out less effort for a given distance traveled. And as Nick said the long boat will ride better in waves. So if it were me I would be playing around with designs in the 16-18 foot range probably.

I linked to this a couple of times but a couple of years ago I wanted to see what effect length has on a boat. I always heard longer is faster but I just knew that there was more to it. So I did a little study and then created a web page to show what I found. Maybe it will help you decide what you need.

http://www.kudzucraft.com/articles/longboatmyth.php

And maybe someone has so info on this, but I am pretty much convinced that 'glide' is directly related to the hull resistance. Wind and waves will obviously have an effect. But the slippery hull is going to have a better glide.