I'll amend my previous post - but in doing so I've discovered something else about iFly. One can do the tap and hold (or what works for me is press and hold) - on an airport that is designated in the active flight plan as a landing location and the choice to extend runways is present.
However tapping/pressings and holding on any other airport - whether in the flight plan or not (as long as it is not designated as a landing waypoint) does not bring up the option to extend runways.
FWIW, this was tested on ver .18 using stock Android 8