The easiest way to get to Hintertux from Mayrhofen is by using the 4104 Bus.
The bus picks up at the Bahnhof, as well the Post Office, Penkenbahn, Rauchenwald and Stillupklamm. On the way back comes along the main road and will stop at the Spar and Bahnhof. If you want to get back to the Penkenbahn, then it is a short pause at the station and it will go around again.
The timetable can be found here.77