Someone hiking the John Muir Trail (~200 miles in the Sierras) will average around 15 miles a day. Sometimes, people hiking the Pacific Crest Trail (1000+ miles) will do 30, but man, that's a lot of miles!! Even for people who are experienced hikers who have trained substantially for that kind of thing. Even if you're generally fit, normal people still need to train, because your bones can't handle that otherwise.
Granted, Sam and Dean are demonstrably not normal people. True to brand, they are Built Tough, just like the Chevy Silverado. These are the people who 1) ran around for a year in a monster-ridden forest and still took the time to shave, and then 2) walked out of the 100-Mile Wilderness in Maine, and also 3) got shot by a werewolf, nearly bled out, got strangled, stalked and killed two werewolves, hiked to a car, drove a car, and then killed some more werewolves. It's not that I doubt them. I just loved that "walk to Dayton from some amorphous point in NE Kentucky" felt like a completely reasonable plan to both of them. XDDDD
Also, they had to cross the Ohio River at some point--unless the point of that tunnel was to avoid the river (but it sounded like it went through a mountain?).
1) Someone please write the overly-technical tag where they carry out this river crossing. Caulk the wagon and float? Ford the river? Apocalypse bridge?
2) If we decide that the tunnel also cut under the river (and through a mountain), that puts it still 50-80 miles outside Dayton, which would have been 50-80 miles without Sam. D;