nathank, I'd imagine your average speed has to be a little slow because of all the pedestrian and other bike traffic on the trails -- along with 90 degree turns. If this is the case, that's a lot different from my commute which is all roads and the only thing stopping me is red lights. But the scenery along your route is great, which is a lot different from mine!!!
yes, the route along the river i have to ride a little more slowly with curves and other riders and the occaisonal pedestrain who doesn't stay off of the bike path... but here i could take a faster route, but i like to ride along the river... although i get to bypass ALL of the traffic lights and stop signs so it might not be that much slower overall.
but yes, for the latter part of my commute i ride MUCH faster as there are fewer people and the then i ride on the street.