If anyone's looking for a 5 mile mostly off-road walk, followed by a return to your start by rail, you can do worse than start at Newport Station, look out the old canal towpath (it's not intuitive as to where you find it, mind...). Once you've picked it up, head, eventually, for Rogerstone.
At the start, you'll find a canal tunnel with only one end (the other end vanished behind concrete). Later, you'll pass Allt yr ynn - where a poet allegedly wrote a very well known poem as well as one less well known one. It isn't quite the setting that it was, as the lock there is now alongside the M4. Then, you'll pass a major flight of locks, after which the canal, more or less nailed to a hillside, takes you onward to pass within about a mile of Rogerstone Station. Dig out the likes of OpenStreetMap for this bit, as there's a couple of footpaths that cut the hike from the towpath to the station.
https://www.openstreetmap.org/search?query=rogerstone#map=17/51.59713/-3.06300