Originally Posted by GLAtrans

I mapped it out for clarity (ignore the dodgy "Terminal 5 london heathrow airport" legend, that's the Sofitel). Google's giving it 4.4 miles one way, around 7 km. You can always just return straight back to T5/Sofitel from that roundabout on Stanwell Moor Road, take the bus/cycle bridge over to the Western Perimeter Road.