There is track in places. First on one side, then the other. Sometimes the track disappears and becomes a road. Many times there are gates when a bridge/street crosses the route. It's better downstream from Horikiri station than it is upstream. Some sections are really nice.
Hmmm sounds a bit like a PIA. I'm trying to work out a good summer commute from Nerima to shinjuku, down Arakawa, maybe a few palace laps, ideally if there was a good route down sumidagawa it would be perfect. Going all the way around Arakawa might be a little too strenuous.