The river is definitely slow and easy, but the distance you can go is limited by your willingness to portage (didn't seem like it would be too easy), or ability to slide/wiggle your kayak over sunken trees. MANY sunken trees. You can probably paddle close to a mile upstream before encountering too many obstacles. Of course, in the spring with higher water, there might be more clearance over most of the fallen trees.
Without pushing too hard, we had time to go about 2 miles upstream to Auburn Street...the site of a former bridge. We pulled the kayaks off to the side here, and went up to explore and eat our lunch in a nearby field. As the sun was getting low, we headed down to the kayaks to head back. A word of warning: the return trip takes you west...into the setting sun. This can be lovely, but a bit tricky when trying to see the proper path over the downed trees.
Ironically, most of the wildlife we caught sight of was within 20 feet of the parking lot on our return trip...a small flock of ducks and a great blue heron. Other than that, there were only a few hawks seen flying overhead, some small birds, and gray and red squirrels jumping in the branches.
The whole paddle, with a lunch stop took under 3 hours.