Jim, I have a CH17lt and I've raced it several times including the Blackburn race in MA. It's about 19.4 miles and my average was 5.23 mph. Some of this was moving with the current up the tidal Anisquam river, so I think your speed is pretty good.

I like the tracking ability of the design so I can paddle hard without really worrying about my course, and it's easy to turn when you need to.

Jim,

 if you look at the following link, it gives results in place order and total elapsed time from the 2009 race  http://kayak-adventure.net/L2L_Race_Results.htm   first place is 1:47 in an epic 12 (a racing machine)  45th place through 70th place all finished within about 20 minutes of one another from 2:30 to 2:55.  so you would have finished in the fat part of the pack in boats all over the map.  the course has a 4 hour time limit....and your time certainly doesn't come close to having to have you towed off the field.

so while your potential performance in absolute terms isn't going to win the race...it is very respectable....and so my reaction would be ....you should be proud of what you have done in practice and what you have the potential to do in the race.

but i think another way to look at this is how you simply improve relative to your own performance over time. 

holding the equipment constant, my experience is that technique has a huge impact on performance in any long race. and while you haven't identified your approach to training, in addition to building your strength and stamina, a good paddling technique video like the Forward Stroke Clinic by Brent Reitz (see the CLC book section) or a paddling coach who has race experience can do wonders in improving your personal performance. (do note, these guys probably aren't going to optimize you around a greenland paddle though....but most of the core concepts are universal to various paddle types)

outside of your technique, equipment variation is also important to absolute performance.  the c17lt is a solid, fast, performance craft.  but there are faster, easier pushed racing hulls....but without changing everything, i think you can do very well in the clc17lt platform. 

the paddle, of course is critical too.  and while i love a greenland paddle, i would be candid with you that it is not typically conceived as a "performance" paddle by any stretch of the imagination.  so, to the extent that you wanted to play with your equipment, i think i would look at your paddle as an important piece of equipment that you would want to consider changing as well if you have high performance expectations.

anyway, i hope those thoughts help.  just my 2 cents.

 enjoy and good luck in the race.

 howard