How can you use daily walks for spiritual growth in Diamond Way Buddhism? For a variety of reasons, my daily routine will include two one-hour walks per day during the coming couple of months (one in the morning, one in the evening). I want to use that time for my spiritual growth. In some branches of Buddhism (Zen) there is a walking meditation.

I'm practicing Diamond Way Buddhism and I'm wondering if there are any exercises in that tradition I could do while I'm doing those 2 walks per day.
From how I was taught, in Zen the best way to use daily walks for spiritual growth is to not use them for spiritual growth ;) In the true spirit of Parjnaparamita, forget about "spiritual growth" which is a very deluded motivation, and just walk.

From Karma-Kagyu perspective, I guess we can get into more details...

When you walk, it is very important to feel your entire body -- and not ignore any tension or tightness. The tension manifests in your posture and your gait, in how you hold your head-and-shoulders, your torso, your pelvis, how you put your weight down on your feet, what you do with your hands. When you feel comfortable, you can open up and your body parts can move more freely and be in harmony with each other. And vice versa - when you make your body and feet conveniently positioned - then your mind can feel comfortable. Imagine how a king or Buddha would walk, to us it looks like they walk "with pride". The reason they walk like that is because they always keep themselves very comfortable. They don't maintain negative stories and they don't maintain body tensions.

As you walk and see things, try not get caught up in stories. When you see something, if you habitually relate to it as "good" or "bad" then you no longer see it, instead you see your thoughts overlayed on top. Usually when we judge we judge on the basis of one or few "signs" or "marks" (nimitta - characteristic features that give us hints about qualities of the object). We then make inferences based on those signs and interpret what they tell us. This formula "marks => qualities => story" is what my last teacher called "preconception". They (preconceptions) come from our past experience. We then compare that story with how things are supposed to be in the perfect case. And then we evaluate it as "good" or "bad" - and we feel corresponding emotion. When this happens we no longer see things as they are, we only see the "signs" and the "story" colored with some feeling. Instead, when you don't let this mechanism go on autopilot, then you see things more fully - you see many other aspects than just signs, your experience becomes much more rich and nuanced. In Buddhism we say you see things "as they really are".

Same thing happens if seeing some things or some people makes you judge yourself. This happens in two cases. Either you see something or someone that you evaluate as "very good" - and then you start comparing it with yourself and come to a conclusion that since you are not "so good" - then you must be "bad". Or there is a case when you imagine how someone must be seeing you from their perspective and you imagine what they probably think about you (awesome/worthless, smart/dumb, handsome/ugly etc.) There are many variations of these - but basically they all involve some comparison and imagination. So again, instead of feeling yourself as being fully yourself - you are making up some story in your head. Needless to say, when this happens you usually lose the sense of your body and posture/shoulders/lower back/pelvis/feet. Instead, when you don't alienate (deny, disown) yourself and you allow yourself to be natural/uninhibited/spontaneous, we say you become "an authentic person" or a "true person".

So the idea with walking meditation is to get out of your head and just walk, walk while seeing the world in all its multifaceted colors, and while feeling you are who you are with all your various qualities - without getting caught in those imaginary stories.

Of course, when you walk you don't need to think about any of this - you just walk comfortably and look around. It's the simplest thing in the world.