My average, and observations
I usually figure 3mph at the end of the day, i.e., after stretch breaks, pull around a strainer or two, etc. If I go much faster, it usually involves a good current, and I sometimes go slower if strainers or swims per mile rise.
Observation: I sometimes say "ax" when I emmulate folks from parts of the country stereotyped as uneducated. Stereotypes always fail at some point.
Observation: Was using mapping from Switchboard.com to look at a river recently. I clicked on "birdseye view", having no idea what it did. Try it! You can do a detailed check out of your river.
Observation: Rivers seldom go straight. Roads sometimes, but not always do. I can cite many instances of shuttle routes that are many times the distance of river miles. But it is usually the other way around.
Agree with posters that suggest getting a little boat time under your belt before tackling a 60-mile route. If you break down cycling, you are by the side of the road. On the river, you can be in an uncomfortable situation where recovery is more difficult. You probably won't die, but you might have some discomfort. What for? It's "spoozta" be fun.
Have you checked the places2paddle section of this site? Might find some info on your river there? If not, be sure to add something when you come back. Also, consider hiring a local outfitter to shuttle you or your vehicle from one end to the other. Local outfitters probably know a lot about the river you are targeting. Sometimes outfitters are reticent to share info with callers, but if you are a customer will be forthcoming.
Good luck,
~~Chip