You can twist to brake with skis too! (hockey stop) :)

You shouldn’t be exerting too much muscle if you are skiing on relatively unbroken terrain. The only time you really use a lot of leg muscle is in deep, deep, heavy powder, in order to keep your tips up. Or if you are going over huge moguls or bumps at a high rate of speed, using your legs as giant shock absorbers.

If you have tight boots, sharp ski edges, and a slightly forward stance, your leg muscles shouldn’t tire out very quickly.