Even if your in decent shape, snowboarding uses specific muscles, and if you don't snowboard often those muscles don't get used that much. Bicycling helps because it targets those thighs, especially if you do stand-up hill climbs. It depends on the person too, but along with reducing the stance width, try a less angular stance. Having a 16' duck both ways kinda pushes both feet out. Try something like 15/-12 or 15/-9 or 12/-9, and see how that feels.
PowderHound and TreeNinja