Runways on runways
A question regarding runways that are marked on top of bigger and wider runways.
I was at a very friendly location yesterday, first visit. The runway is marked out very clearly on top of what is obviously a much longer and wider original runway. Being as it ws my first departure from there I like to make sure I'm doing it right, so I inwardly digested the information on the clubhouse notice board re departing. It called for a taxi down the edge of the marked runway (bags of room) to a holding point and do your engine run up there before departure. While I was there running the engine a call came from an a/c on finals asking me to clear the runway as they were on finals. My immediate thought being 'I'm not on the runway, the runway is marked out and a fair way over yonder.' I did however taxi onto the grass and cleared the actual tarmac bit. It was not a problem whatsoever, all very friendly but it set me thinking (always dangerous).
When is a runway not a runway? Think of somewhere like Fenland or Breighton where the taxiway is simply the wider bit of the runway, the runway itself being marked, much as the situation yesterday. You can't taxi off of Fenlands taxi strip because you will be in a ditch. So what is the actual definition of a runway and when are you actually on it?