Wall sits are a form of muscular endurance and will improve that quality fo sure. Mountain runners should work mainly on single legged exercises though as this is how you will be using your body in the mountains. Two legged strength exercises remove the need for hip stabilization. There are better single legged ME exercises that wall sits. Try sets of 10/leg weighted box steps minimizing the rear leg use and rest 30 sec /set. Try weighted front lunge sets of 10/leg with 30 sec rest. These will burn your legs AND be much more sport specific.