You can start the hike either from Namaze or Takedao, both are connected to the JR line and have trains to Osaka, although they are both only serviced by local trains so don't stay on the express past Nishinomiya-Najio station or you'll miss it. I've done the hike both ways and think the Takedao end has the best views so if you start from Namaze you've got something to look forward to, although the whole route has pretty fantastic scenery so it doesn't really matter either way.
The hike takes about 3 hours but being an old railway line means that the whole thing is pretty level, albeit bumpy, so basically anyone who can walk shouldn't have too much trouble. I'm a mildly overweight heavy smoker and managed it no problem. I've seen a couple of people do it on bikes as well but I wouldn't recommend it unless you have super suspension.
It doesn't take long before you come to the first of several tunnels that you have to walk through, some of which are pretty long and pitch black so make sure your phone is charged for the flashlight or alternatively bring an actual flashlight/torch. I guess it depends on the time of year/day but the first time I went we didn't see another living soul at all and the second time the tunnels were full and lit up like someone had released hundreds of gigantic Japanese speaking fireflies. I kind of liked the effect of all the torches glinting off the water in the tunnels plus it made the fear of being eaten in the dark by something seem less probable.
That's pretty much it, I don't want to spoil it by going into every little detail. It's a nice hike and if you have a free morning/afternoon in good weather it's definitely worth a visit.
Things to bear in mind:
- There aren't any shops or restaurants nearby so bring whatever you'll need to survive for three hours of walking.
- There are toilets at either end but nothing in between, unless you count behind a tree in the forest, so go before you set off.
- Both Namaze and Takedao station can only be accessed via local trains, the closest express stop is Nishinomiya-Najio station.
- Not that I've seen any myself but there are wild boar, monkeys, and apparently bears in the mountains so just keep an eye out for them. There are also snakes, which I did see one of today.