Hey Jude (hey, isn't that the name of a song ;o)
The walk to the river is the roughest part of the trip. Wear some water shoes for sure. It is about a 30-40 minute walk once you start and you're carrying your tube. It wasn't too bad as our guide stopped a bit and told us things about the trees, etc that we passed by.
One thing I will tell you is, once you get in the water, keep your "bum" up. It tends to get shallow at times. At least it did, when I went last March.
Enjoy!! And like I said, "bottoms up".