It’s literally showing you how far they’d be willing to walk. The center is Scotland (where they’re from), and the first circle is a 500 mile radius from there (“I would walk 500 miles…”).
The second circle is a radius of an additional 500 miles (“…and I would walk 500 more”) - the graphic is showing exactly how far The Proclaimers, well, proclaimed they’d walk.
That’s the joke.
Now you are mansplaining.
CD