what's your source for distances working differently - without trawling through loads of pdfs this was all I could find on the shadow plane
The Shadow Walk spell lets you walk at 50 miles per hour [material plane] while walking normally on the shadow plane.
Plane Shift makes distance irrelevant.
/cevah
Are you sure you're not thinking of the First World?
The First World is essentially your Pathfinder Narnia, where you can go in as a child, grow up and have years of adventures and emerge just minutes later. Entering through one entrance, walking for five minutes and exiting through another exit could mean you emerge hundreds of miles and hours, days or years later. The First World also doesn't resemble the Material Plane in any meaningful way.
The Shadow Plane is instead a black and white counterpart to the Material Plane. Travel is typically expedited but occurs at a fixed rate, not variable such as the First World. Time also passes normally on the Shadow Plane, and geography mimics the Material Plane roughly, though precise measurement is impossible.
In short, if you're looking for a way for your players to cross large distances in the blink of an eye, the First World is a great way to get them there quickly. Just watch out for Fey!