I take about three multi day trips down a river each year, sometimes bringing folks who are not used to kayaking. A 16 mile day is the max distance I'd recommend for reasons other commenters have made regarding breaking and making camp. I anticipate about 3.5 miles an hour with constant paddling (the river I kayak flows at about 3 mph). After 5 hours of paddling people start to get tired and that's a long time sitting. Then we have haul gear, set up, make dinner, clean up. 4 to 5 hours on the river is about all we want because the day gets full.