I'm getting a bit confused by the various design characteristics, and I have a question about long distance touring.

I've been thinking about doing a several week paddle down the Mississippi river next year. Before doing something quite that long, I was planning an initial trip down the Illinois from Chicago to St. Louis (~300 miles) to make sure I don't discover on day 2 "OMG I HATE THIS AND I HAVE 7 MORE WEEKS."

The goal for the longer trip would be to log a lot of miles every day week in, week out on mostly flat water.

I'm usually in the 170-175# range, am fit, and am faster in my slow boat than most recreational paddlers in their long boats. (I'm also aware that there are a lot of guys out there much, much faster than me, so believe me I'm not bragging).

Cargo capacity isn't a big deal since I would be passing cities daily and I don't mind packing light or having a skinny boat.

I see two ways of looking at this. The first is Brian over at Cape Falcon, saying the tradeoff in drag at longer lengths means the long boat is faster, but it doesn't matter because your average speed is going to be relatively low over very long distances. So why not paddle a shorter boat that handles your average speed more efficiently.
http://www.capefalconkayak.com/choosingakayak.html

"Sixteen feet is a common waterline length for a touring sea kayak, this will allow a strong paddler to cruise at 5 miles per hour, it also adds a lot of volume for camping gear. However, I have found that most people aren't doing multi-week trips at 5 miles per hour. A much more common use of a sea kayak is day and weekend trips at 3-4 mph. For this sort of use a 13 foot waterline will be more efficient and feel faster, and be more fun to paddle. If you want to test this a gps can be used in conjunction with a heart rate monitor to calculate calorie for calorie efficiency. I am not an athlete, but I am a strong paddler, I paddle 3 times a week and I have never exceeded 4.5 mph under normal cruising conditions, even when paddling with strong friends or trying to set personal speed/distance records. My average cruising speed is 4 mph. (3 mph upwind, 5 mph downwind.)"

The second seems to be what I see borne out in the market, which is kayaks like Epic 18's (nearly 18' waterline) or 18' British boats (16-17' waterline) distance touring, even if they aren't using all the displacement for cargo. This would suggest long waterline boats are faster in the short distance and long distance. Or they're faster for people like Freya and Greg Barton, so everybody does what they do, and they end up doing the equivalent of putting they four cylinder engines in big ass cars.

The third is I may be thinking too much about this. I asked Nigel Foster the length question up at the WMCKA symposium, and he didn't feel there was a particularly significant difference. He uses the longer boat when he has more stuff to carry, but might use it otherwise as well. He seemed to suggest that over a long distance, the glide advantages of the longer boat might cancel out the drag advantages of the shorter boat.

Anybody have thoughts? If you were on a long distance trip and packing light, what boat would you build?