Walking up hills is as good for exercise as riding a bicycle. If I am not feeling up for it I hop off my bike and walk up steep hills all the time, especially on long rides where I want to keep my energy up. Olympic runners in training will often stop running when they get to a hill and walk up it as both activities can keep the heart rate the same. 80%+ of an olympic athletes training is kept at a lower heart-rate and effort, if they max out their effort every day then they are actually making no progress or even going backwards.
I had the typical close-minded cyclist stop and ask me why I was walking my bike up a steep hill, they thought the only reason for it was something must be wrong, but the only thing wrong is their lack of education. When you see a really serious and well-educated cyclist, they will be taking it easy most days you see them riding.