In reply to MG:
It's worth training to walk really quickly. Learn to stay reasonably tall and keep a brisk rhythm rather than just gradually folding up until you're lying on the hill. In my experience, concentrating on using your glutes more than your quads, and on flexing your ankle to get a flat foot where possible, will allow the walking to give your quads and calves a proper rest.
Since (as you pointed out) you do have to walk for long periods on some routes, it makes sense to get good at it. It also creates more overlap between walkable and runnable terrain, so you can vary things more easily according to fatigue, will and the need to take care of eating/drinking etc.
I don't think there is a rule, but the more confident you are that you can move efficiently in both modes, the more you will make choices based on general tactics rather than sheer necessity!