Three short lines, and a lot of tiredness packed into them. The speaker is out on a long beach walk, and the first two lines turn distance into time: the day lasts exactly as long as the way back home. That's a quietly weary thought. The walk has stopped being a pleasure and become something to get through, with the return trip stretching the whole day out.
Then the last line interrupts with something blunt and unglamorous: a gull vomiting. We usually expect a beach poem to give us crashing waves or a glowing horizon. Instead the speaker notices something ugly and a little gross, and the poem just leaves it there. The mood is flat, dry, and slightly funny. The gull's sound seems to match how the speaker feels, a bit queasy or fed up. Judging by the imagery, the poem finds honesty in refusing to make the walk pretty.