I have begun adventuring where I drive to a boat launch on a river near a bike trail, put the canoe on a trailer, and tow the canoe up the trail with the bike. At another boat launch I put the canoe in the river and the bike in the canoe, and paddle back down to my vehicle. I find that I make about 5 miles an hour pedalling up, and about 3 miles an hour paddling down. This limits a day trip to about 15 miles; 3 Hours up, and 8 hours down plus drive time, lunch, launch, etc.
I am currently working on the section of Monongahela and Youghiogheny Rivers between Connellsville(PA) and Pittsburgh PA? The trail and goes farther up stream but at Connellsville the river is closed to open canoes, 63 miles down stream the bike trail ends(Google map)
I used the bike trail feature of Google maps and followed the Ohio to the Mississippi all the way to the Gulf of Mexico. in the 3300 river miles between Connellsville and New Orleans, the section I am working on now seems to be the longest stretch.
A little thought on the subject suggest that old rail systems that parallel small to midsize rivers, that have been converted to bike trails will probably offer the longest stretches.
So where is the longest stretch? I would like to go someplace and spend a week or two biking up and canoeing down different stretches of river, where is the longest stretch of bike path that parallels a river I can canoe on?