Owen begins the poem with a description of marching soldiers. His focus is on the grimness and misery of the situation, which seems to have rapidly aged the men and zapped them of life. In the first line, the speaker compares the soldiers to "old beggars" bent under their burdens. In line 2, he compares their coughing to that of "hags," a derogatory term for old women, and emphasizes their physical weariness as they struggle through mud. In lines 3 and 4 he clarifies direction, showing the reader that the soldiers are marching away from enemy territory (marked by the "flares") and towards the place where they will be able to rest.
The first four lines thus set up a scene, helping the reader understand the soldiers' fatigue, their frustration (expressed by cursing), and the constant danger that still surrounds them (represented by the flares). Owen uses consonance to lend a harshness to the sounds of the poem. In the first line, the letter "b" appears in three stressed words ("bent," "double," and "beggars"). This gives way to hard "c" and "k" sounds, with "sacks," "coughing," "cursed," and "backs." Although the "k" sounds of "knock-kneed" are silent, they contribute visually to the hard consonants of this section. The sibilance of "distant rest," meanwhile, makes it stand out from the rest of the landscape, sounding like a whisper, perhaps not entirely real.
These lines are basically in iambic pentameter, a meter that consists of five iambs per line. This sets up the expectation that the rest of the poem will follow this pattern. Owen does play with stress a little, though. He crams more stressed syllables into the first two lines than belong in iambic pentameter.
Bent double, like old beggars under sacks,
Knock-kneed, coughing like hags
This decision sets the rhythm of the poem rocking. It's over-stressed, unstable, reflecting the instability and roughness of the scene the speaker is describing.