The Wansdyke ran for a long way, and probably uninterrupted, all the way to the Severn, but it wasn’t an all-earthen barrier; a couple hundred years ago its complete route could still be traced all the way to the river, now those traces are gone (maybe Time Team looked for some of it with their geophys?). As you said, it was post-Roman, and got that name because the Saxons regarded it as a work of Woden. Along with the probably pre-Roman Grim’s Dyke, parts of its route can be seen on this map off one of those “real King Arthur” blogs:
Offa’s Dyke used to be attributed to the Mercian king Offa, he apparently made use of it. A shorter earthwork called Wat’s Dyke faces it along a small part of its length, and used to be believed to have been built in response to Offa’s.
Since so little was known of Wat’s excavation was undertaken, and wood from its base was RC dated to a period a few hundred years before Offa’s Dyke. Ten years went by.
Someone started wondering if all was really known about Offa’s Dyke, so it too was excavated and RC dated.
Offa’s Dyke turned out to be a few hundred years older than Offa.
The romantic fool in me attributes Offa’s Dyke and the Wansdkye to the leader/dynasty now dubbed King Arthur; since some of his 12 battles were also in the north, the proto-Arthur would also be responsible for the post-Roman use of Hadrian’s Wall, and would have been based in the original Roman east. Colchester was Camulodunum to the Romans, which is easy to transliterate to Camelot...
Someone should write a book on all of these. If I remember right, there was even a Corduroy road of logs somewhere in VERY early Britain.
