We mentioned that these guys seem a bit otherworldly before, but we'll say it again.
Notice how lines 5-6 collect lots of "l" sounds? Words like "lost" and "limped" and "blood" all roll on our tongues, making the experience of reading the lines seem even lllonger.
It's all part of Owen's technical dexterity: he's trying to get us to feel how interminable the soldiers' march seems right now.
Also notice that the blood that has been shed seems to clothe them now, (or at least their feet). This creates a vivid image suggesting that the war – figuratively and literally – is enveloping their very beings.