Terrain and tires are going to have a huge impact on your time. Knobbies will play havoc with your speed, especially on descents. Personally, I have found that having slicks on a mtb is the best thing you can do to improve speed.
Terrain has a big impact as well. During the Tour of the Litchfield Hills, I averaged 11.1mph over a 77 mile course. The course had 3 700'+ climbs which played havoc with my borderline uberclyde physique. Take the climbs out, and I'm averagine 13 mph easily. Put them back in, and I'm lucky to break 11.
Also, I'd consider trying to do something during the boat ride to prevent muscles from seizing up during the break. Bring some ben-gay, keep walking, whatever. Being off the bike for 20 minutes won't slow you down much later. Two hours might be enough time for aches and pains to start taking effect.