How far from a small single runway airport's taxiway, can something like a walking or biking trail be built?
ICAO recommends somewhere between 16.25 and 57.50 metres from the centreline, depending on the type of aircraft that use the taxiway (code letter A-F).
From ICAO Annex 14:
3.9.8 Recommendation.— The separation distance between the centre line of a taxiway and the centre line of a runway, the centre line of a parallel taxiway or an object should not be less than the appropriate dimension specified in Table 3-1, except that it may be permissible to operate with lower separation distances at an existing aerodrome if an aeronautical study indicates that such lower separation distances would not adversely affect the safety or significantly affect the regularity of operations of aeroplanes.
3.11.1 A taxiway, other than an aircraft stand taxilane, shall be included in a strip.
3.11.2 Recommendation.— A taxiway strip should extend symmetrically on each side of the centre line of the taxiway throughout the length of the taxiway to at least the distance from the centre line given in Table 3-1, column 11.