My walker is working now.
It is somewhat slow, as it took 2:18 to reach the finish-line. There is also a slight clipping problem because the legs are bending, but is has fully funktional tank-controlls including turning in the spot (thanks to two KALs as suggested by @Lisias).
I will probably redo it with an additioal hinge istead of the piston to get rid of the clipping, but it won't get much faster as there is no way to increase the craft's speed whilst moving and starting at a higher speed just causes it to flip over.