This was a nice thread to read. I like the notion of sport as competing against others, on a somewhat organized and official basis. I am hard pressed to think of any sport that doesn't do this, so it's a useful definition.
Keep in mind, prior to our car/horse buggy days, walking was pretty much it, and walking is what our bodies are designed to do. So backpacking is just doing what is most fundamentally natural for the human body. Now of course, there are some people who have to force totally natural activities into some more agro box so they can justify it to their hyper competitive personalities, so I like to leave adventure racing to them, and enjoy the trails and natural environment while they go elsewhere.
In a sense, ultralight backpacking at its best is actually returning us to the base weights that were used by travelers long ago in general, barring actual moves and so on, and that's what it feels like to me now walking with that weight, pretty natural, not weird, not a struggle, like a 50 pound pack felt.
If you think native americans, they had massive networks of trails, and they walked around on them, that was how you got around back then, and that's really all we are doing now when we backpack. Then, when they wanted to compete against each other, they had sports, just like we have today, and the difference was the organized competitive aspect, ie, there are winners and losers in sports, runners up, and so on.
I've done sports, bike racing, and so on, those are totally different, I'm competing against others there, and the point of the training is to compete successfully, or as successfully as I could. Training for backpacking has some of the same elements to me, but it's just taking walks in the end, albeit a bit more vigorous and focused than I'd normally do.
I really like the feeling of walking all day, but that's because that's so natural for the body, it's what it's built for. Driving and all that is the weird unnatural activity.