Interesting, always wondered why runways have weird declared lengths. One of the places I fly from is Gamston which has a declared length of 1199 mtrs on a runway of 1683 mtrs.
Probably why Old Wardens extension is 150 mtrs as well.
One of the reasons that runways have displaced thresholds is because of approach onstructions which may be why 03 at Gamston has. AD 2-10 shows it has trees on the approach. Note Pooleys quide doesnt show approach obstructions another reason for using the official AIP for flight planning!
At a licensed airfield there must at least be a pattern A holding point for each runway at a safe distance from that runway. Technically if you taxy over that its a runway incursion, so you can consider any area the other side(runway side) of a holding point as the runway. Even a strip alongside the main marked runway is technically still the runway and after landing you have not technically cleared the the runway until you have cleared the pattern A holding point.
At an unlicensed airfield who knows!
Running up on old non maintained tarmac may damage your prop, something to consider if on a loose surface.
To my mind its always safer and wiser to face the approach for runups. We/I ban turning into wind below 20 knots and below 20 degrees temp