Start at the Olympic Stadium in Partenkirchen (dating back to 1936). Then run up the Partnach, avoid the toll booth for Partnachklamm and take small detour around the Klamm (as shown on GPX track). The detour will take you back to the Partnach above the Klamm where the ascent to the Schachenhaus via Kälbersteig starts. The Schachenhaus (completed 1872) is a less well know (and much less visited) gem of King Ludwig II, in ca 1800m above see level and with stunning views. There is no cable car going to the Schachenhaus, so not many tourists here. The Schachenhaus is also accompanied by an alpine botanical garden. More information about the destination can be found here.
The GPX track is only one way. Is it up and back down to the start? Would be good to update the gpx then.