Yes, the amount of attention that any activity requires is hugely variable, depending on just the sorts of variables you mention.

Even so, I find there's a kind of magic in being fully embodied in my exercise, paying attention to how I put my feet down, how my ankle flexion changes with the grade of the trail, how I hold my head, etc.

Even the most routine, completely repetitive exercises, like squats or push-ups, benefit from paying attention to which muscles you're using, and how that changes over the range of motion of the exercise. At least, I've seen a lot of benefit in my own practice—more power, more control, fewer injuries, a sense that there is a "graceful" me that never presented himself during my youth, but that I've managed to capture some glimpses of since I've started doing this.