As far as i know, Unless there is a road or a footpath directly between two points the routing engines seem try use a way ( Path or road) that it close by and me may leave a gap between point and the way. so distance will i think be not much use over very short distances. OSRM and Graphopper give reasonable distance measurements in KM over larger distances but do not seem to be useful for this case. But i may be incorrect! see this:-https://www.openstreetmap.org/directions?engine=fossgis_osrm_foot&route=52.3284%2C-0.1772%3B52.3278%2C-0.1728