I can't speak of the walking path but the Hadrians Wall cycle route is far from "fairly flat" in the middle bit, although extremely beautiful.

Some people of my acquaintance did the walk in a couple of groups three years ago and I took a group on bikes to do the ride, theres a blog of the walkers five day effort out there somewhere if you want me to find it, its got their camp site info and stuff like that on it.

My advice would be to do it on a bike - two days one stop over



Not sure how the dog would get on though.

"Fairly flat" reminds me of when I used to choose my next running event from the back pages in the running magazines.
Each race was described in abbreviations, one of which was "U" which stood for "Undulating".
Some of those "U" routes felt more like hilly to me.

Doubt his legs would reach the pedals, he'a jack russel cross

Scooter, with shortened handlebars.

Yes his red one would be perfect, I'll have to pump the tyres up though