A good friend of mine (PA Hillbilly) used his last summer on a 6 hour float down the James River. It helps if one, you have some prior knowledge of the area and two, if you know how to read a river's flow and riffles versus rapids, so you can know when to pull your mirage drive versus just opposing the fins and locking them using the retaining bungee. It will save you money in the long run. When in doubt pull it.
Before I moved to south Georgia in 2006, I used to float the James River starting at Eagle Rock for about a 5.5 hour float. Great strecth of river to fish for small mouth bass!